Issue 4015: Add \magnifyStaff.

* Add \magnifyStaff.
* Rewrite parts of the \magnifyMusic function so that it can share
  some scheme code with the new \magnifyStaff function.
* Add regtests.
* Update documentation.

11 files changed, 345 insertions, 100 deletions

diff --git a/Documentation/essay/engraving.itely b/Documentation/essay/engraving.itely
index 5040f2771b..5136a331e0 100644
--- a/Documentation/essay/engraving.itely
+++ b/Documentation/essay/engraving.itely
@@ -447,13 +447,9 @@ magnified by 236% to print at the same size as the previous example:
 At smaller sizes, LilyPond uses proportionally heavier lines so the
 music will still read well.
 
-@ignore
 This also allows staves of different sizes to coexist peacefully when
 used together on the same page:
 
-@c TODO: are the stems in this example the right thickness? How should
-@c line weights be scaled for small staves?
-
 @c Grieg's Violin Sonata Op. 45
 @lilypond[indent=1.5cm]
 global = {
@@ -464,9 +460,7 @@ global = {
 \score {
   <<
     \new Staff \with {
-      fontSize = #-4
-      \override StaffSymbol.staff-space = #(magstep -4)
-      \override StaffSymbol.thickness = #(magstep -3)
+      \magnifyStaff #2/3
     }
     \relative c' {
       \global
@@ -503,7 +497,7 @@ global = {
   >>
 }
 @end lilypond
-@end ignore
+
 
 @node Why work so hard?
 @unnumberedsubsec Why work so hard?
diff --git a/Documentation/music-glossary.tely b/Documentation/music-glossary.tely
index a58b49971b..056d6b0064 100644
--- a/Documentation/music-glossary.tely
+++ b/Documentation/music-glossary.tely
@@ -3618,8 +3618,7 @@ played above the bass notes.
 @lilypond[quote,line-width=13.0\cm]
 \new GrandStaff <<
   \new Staff = "rh" \with {
-    fontSize = #-3
-    \override StaffSymbol #'staff-space = #(magstep -3)
+    \magnifyStaff #2/3
   } \relative c'' {
     \clef treble
     \key es \major
diff --git a/Documentation/notation/spacing.itely b/Documentation/notation/spacing.itely
index dd8ef02293..c88f6b2c1b 100644
--- a/Documentation/notation/spacing.itely
+++ b/Documentation/notation/spacing.itely
@@ -1283,24 +1283,17 @@ block:
 @end example
 
 @item
-To set the staff size for a single staff within a system, set the
-staff's font-size and staff-space using units relative to the
-score's default staff size.  Using relative units prevents the
-proportion of staff sizes from being altered if the default staff
-size is modified with @code{set-global-staff-size} or
-@code{layout-set-staff-size}.
-
-For example, traditionally engraved chamber music scores with
-piano often used 7mm piano staves while the other staves were up
-to 5/7 as large (if space allowed), or down to 3/5 as large (if
-space was cramped).  To achieve the 5/7 proportion, use:
+To set the staff size for a single staff within a system, use the
+@code{\magnifyStaff} command.  For example, traditionally engraved
+chamber music scores with piano often used 7mm piano staves while
+the other staves were typically between 3/5 and 5/7 as large
+(between 60% and 71%).  To achieve the 5/7 proportion, use:
 
 @example
 \score @{
   <<
     \new Staff \with @{
-      fontSize = #(magnification->font-size 5/7)
-      \override StaffSymbol.staff-space = #5/7
+      \magnifyStaff #5/7
     @} @{ @dots{} @}
     \new PianoStaff @{ @dots{} @}
   >>
@@ -1314,8 +1307,7 @@ could use the following form:
 \score @{
   <<
     \new Staff \with @{
-      fontSize = -3
-      \override StaffSymbol.staff-space = #(magstep -3)
+      \magnifyStaff #(magstep -3)
     @} @{ @dots{} @}
     \new PianoStaff @{ @dots{} @}
   >>
diff --git a/Documentation/notation/staff.itely b/Documentation/notation/staff.itely
index 0fa4f35fac..d2bb608d32 100644
--- a/Documentation/notation/staff.itely
+++ b/Documentation/notation/staff.itely
@@ -608,9 +608,7 @@ only a few ossia staves are needed.
     \new Staff \with {
       \remove "Time_signature_engraver"
       alignAboveContext = #"main"
-      fontSize = #-3
-      \override StaffSymbol.staff-space = #(magstep -3)
-      \override StaffSymbol.thickness = #(magstep -3)
+      \magnifyStaff #2/3
       firstClef = ##f
     }
     { e4 d f e }
@@ -632,9 +630,7 @@ example.
   \new Staff = "ossia" \with {
     \remove "Time_signature_engraver"
     \hide Clef
-    fontSize = #-3
-    \override StaffSymbol.staff-space = #(magstep -3)
-    \override StaffSymbol.thickness = #(magstep -3)
+    \magnifyStaff #2/3
   }
   { \stopStaff s1*6 }
 
@@ -671,9 +667,7 @@ break.  For more information about
   \new Staff = "ossia" \with {
     \remove "Time_signature_engraver"
     \hide Clef
-    fontSize = #-3
-    \override StaffSymbol.staff-space = #(magstep -3)
-    \override StaffSymbol.thickness = #(magstep -3)
+    \magnifyStaff #2/3
   } \relative c'' {
     R1*3
     c4 e8 d c2
diff --git a/input/regression/magnifyStaff-bar-lines.ly b/input/regression/magnifyStaff-bar-lines.ly
new file mode 100644
index 0000000000..4bc357e817
--- /dev/null
+++ b/input/regression/magnifyStaff-bar-lines.ly
@@ -0,0 +1,33 @@
+\version "2.19.11"
+
+\header {
+  texidoc = "Bar line thickness and spacing should be scaled along
+with notation size when using the @code{\magnifyStaff} command."
+}
+
+\paper {
+  score-system-spacing = #'((padding . 4))
+}
+
+example =
+#(define-music-function (parser location mag) (positive?)
+   #{
+     \new Staff \with {
+       \magnifyStaff #mag
+       instrumentName = \markup {
+         \fontsize #(+ 3 (- (magnification->font-size mag)))
+         #(format #f "~,2f" mag)
+       }
+     } {
+       \omit Staff.Clef
+       \omit Staff.TimeSignature
+       s4 \bar "|"
+       s4 \bar ":|.|:"
+       s4 \bar ":|.S.|:"
+       s4 \bar "|."
+     }
+   #})
+
+\example 0.50
+\example 1.00
+\example 2.00
diff --git a/input/regression/magnifyStaff-dots-beamlets.ly b/input/regression/magnifyStaff-dots-beamlets.ly
new file mode 100644
index 0000000000..e4dd1f7a5b
--- /dev/null
+++ b/input/regression/magnifyStaff-dots-beamlets.ly
@@ -0,0 +1,21 @@
+\version "2.19.11"
+
+\header {
+  texidoc = "Dot size and beamlet length should be scaled along
+with notation size when using the @code{\magnifyStaff} command."
+}
+
+music = { a'8.[ a'16] }
+{
+  \magnifyStaff 0.50 \music \music \bar "|"
+  \magnifyStaff 0.71 \music \bar "|"
+  \magnifyStaff 1.00 \music \bar "|"
+  \magnifyStaff 1.41 \music \bar "|"
+  \magnifyStaff 2.00 \music \bar "|"
+} \addlyrics {
+  "0.50  " _ _ _
+  "0.71  " _
+  "1.00  " _
+  "1.41  " _
+  "2.00  " _
+}
diff --git a/input/regression/magnifyStaff-space-alist.ly b/input/regression/magnifyStaff-space-alist.ly
new file mode 100644
index 0000000000..561ac9e72a
--- /dev/null
+++ b/input/regression/magnifyStaff-space-alist.ly
@@ -0,0 +1,38 @@
+\version "2.19.11"
+
+\header {
+  texidoc = "@code{space-alist} values should be scaled along
+with notation size when using the @code{\magnifyStaff} command."
+}
+
+\paper {
+  indent = 0
+  ragged-right = ##t
+  system-system-spacing = #'((padding . 3))
+  score-markup-spacing = #'((padding . 6))
+}
+
+example =
+#(define-music-function (parser location mag) (positive?)
+   #{
+     \new Staff \with {
+       \magnifyStaff #mag
+       \consists "Custos_engraver"
+       \override Custos.style = #'mensural
+     } \new Voice \with {
+       \consists "Ambitus_engraver"
+     } {
+       \omit Score.BarNumber
+       \key d \major
+       d''2 \breathe d'' |
+       \break
+       g'1 |
+       \clef treble
+       \key c \major
+       g'1 |
+     }
+   #})
+
+\markup "0.50:" \example 0.50
+\markup "1.00:" \example 1.00
+\markup "2.00:" \example 2.00
diff --git a/input/regression/magnifyStaff-staff-line-thickness.ly b/input/regression/magnifyStaff-staff-line-thickness.ly
new file mode 100644
index 0000000000..4ff9568d55
--- /dev/null
+++ b/input/regression/magnifyStaff-staff-line-thickness.ly
@@ -0,0 +1,21 @@
+\version "2.19.11"
+
+\header {
+  texidoc = "Staff line thickness should be scaled along with
+staff size when using the @code{\magnifyStaff} command.  Staff
+lines can get thicker than the default, but not thinner."
+}
+
+{
+  \magnifyStaff 0.50 b'1
+  \magnifyStaff 0.71 b'1
+  \magnifyStaff 1.00 b'1
+  \magnifyStaff 1.41 b'1
+  \magnifyStaff 2.00 b'1
+} \addlyrics {
+  " 0.50 "
+  " 0.71 "
+  " 1.00 "
+  " 1.41 "
+  " 2.00 "
+}
diff --git a/ly/music-functions-init.ly b/ly/music-functions-init.ly
index b5eb7feb0f..65b4a6db7a 100644
--- a/ly/music-functions-init.ly
+++ b/ly/music-functions-init.ly
@@ -681,9 +681,9 @@ slurs, ties, and horizontal spacing are adjusted automatically.")
      \context Bottom {
        %% TODO: uncomment \newSpacingSection once Issue 3990 is fixed
        %\newSpacingSection
-       #(scale-fontSize mag)
-       #(scale-props unshrinkable-props mag #f)
-       #(scale-props shrinkable-props   mag #t)
+       #(scale-fontSize 'magnifyMusic mag)
+       #(scale-props    'magnifyMusic mag #f unshrinkable-props)
+       #(scale-props    'magnifyMusic mag #t shrinkable-props)
        #(scale-beam-thickness mag)
 
        #music
@@ -691,13 +691,63 @@ slurs, ties, and horizontal spacing are adjusted automatically.")
        %% TODO: uncomment \newSpacingSection once Issue 3990 is fixed
        %\newSpacingSection
        %% reverse engineer the former fontSize value instead of using \unset
-       #(revert-fontSize mag)
-       #(revert-props (append unshrinkable-props
-                              shrinkable-props
-                              '((Beam beam-thickness))))
+       #(revert-fontSize 'magnifyMusic mag)
+       #(revert-props    'magnifyMusic mag (append unshrinkable-props
+                                                   shrinkable-props
+                                                   '((Beam beam-thickness))))
      }
    #})
 
+magnifyStaff =
+#(define-music-function (parser location mag) (positive?)
+   (_i "Change the size of the staff, adjusting notation size and
+horizontal spacing automatically, using @var{mag} as a size factor.")
+
+   ;; these props are NOT allowed to shrink below default size
+   (define unshrinkable-props
+     '((StaffSymbol thickness)))
+
+   ;; these props ARE allowed to shrink below default size
+   (define shrinkable-props
+     (let ((space-alist-props
+            (find-all-space-alist-props all-grob-descriptions)))
+       (append
+         space-alist-props
+         '(
+           ;; override at the 'Score level
+           (SpacingSpanner spacing-increment)
+
+           (StaffSymbol staff-space)
+           (BarLine kern)
+           (BarLine segno-kern)
+           (BarLine hair-thickness)
+           (BarLine thick-thickness)
+           (Stem beamlet-default-length)
+           ))))
+
+   #{
+     \stopStaff
+
+     %% revert settings from last time
+     %% (but only if \magnifyStaff has already been used
+     %% and the staff magnification is changing)
+     #(revert-fontSize 'magnifyStaff mag)
+     #(revert-props    'magnifyStaff mag (append unshrinkable-props
+                                                 shrinkable-props))
+
+     %% scale settings
+     %% (but only if staff magnification is changing)
+     #(scale-fontSize 'magnifyStaff mag)
+     #(scale-props    'magnifyStaff mag #f unshrinkable-props)
+     #(scale-props    'magnifyStaff mag #t shrinkable-props)
+
+     %% this might cause problems until Issue 3990 is fixed
+     \newSpacingSection
+
+     \startStaff
+     \set Staff.magnifyStaffValue = #mag
+   #})
+
 makeClusters =
 #(define-music-function (parser location arg) (ly:music?)
    (_i "Display chords in @var{arg} as clusters.")
diff --git a/scm/define-context-properties.scm b/scm/define-context-properties.scm
index 2d38188d29..b88c8f59f8 100644
--- a/scm/define-context-properties.scm
+++ b/scm/define-context-properties.scm
@@ -562,6 +562,8 @@ part-combining.")
 @rinternals{Pitch_squash_engraver}.")
      (staffLineLayoutFunction ,procedure? "Layout of staff lines,
 @code{traditional}, or @code{semitone}.")
+     (magnifyStaffValue ,positive? "The most recent value set with
+@code{\\magnifyStaff}.")
      (stanza ,markup? "Stanza @q{number} to print before the start of a
 verse.  Use in @code{Lyrics} context.")
      (startRepeatSegnoType ,string? "Set the default bar line for the
diff --git a/scm/music-functions.scm b/scm/music-functions.scm
index 83cee5ce99..22c6eac92b 100644
--- a/scm/music-functions.scm
+++ b/scm/music-functions.scm
@@ -2404,37 +2404,108 @@ Offsets are restricted to immutable properties and values of type @code{number},
     self)
 
 
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;; The following are used by \magnifyMusic
-
-(define-public (scale-fontSize mag)
-  "Used by @code{\\magnifyMusic}.  Look up the current fontSize and
-scale it by the magnification factor @var{mag}."
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; \magnifyMusic and \magnifyStaff
+
+;; defined as a function instead of a list because the
+;; all-grob-descriptions alist is not available yet
+(define-public (find-all-space-alist-props grob-descriptions)
+  "Used by @code{\\magnifyStaff}.  When @var{grob-descriptions} is equal
+to the @code{all-grob-descriptions} alist (defined in
+@file{scm/define-grobs.scm}), this will find all grobs that have an
+initialized value for the @code{space-alist} property, and return them
+as a list in the following format:
+@example
+'((Ambitus space-alist)
+  (BarLine space-alist)
+  ...)
+@end example"
+  (define (has-space-alist? grob-desc)
+    (ly:assoc-get 'space-alist (cdr grob-desc)))
+  (let* ((grob-descriptions-with-space-alist
+           (filter has-space-alist? grob-descriptions))
+         (grob-names-with-space-alist
+           (map car grob-descriptions-with-space-alist)))
+    (map (lambda (grob-name) (list grob-name 'space-alist))
+         grob-names-with-space-alist)))
+
+(define (magnifyStaff-is-set? context mag)
+  (let* ((Staff (ly:context-find context 'Staff))
+         (old-mag (ly:context-property Staff 'magnifyStaffValue)))
+    (not (null? old-mag))))
+
+(define (staff-magnification-is-changing? context mag)
+  (let* ((Staff (ly:context-find context 'Staff))
+         (old-mag (ly:context-property Staff 'magnifyStaffValue 1)))
+    (not (= old-mag mag))))
+
+(define-public (scale-fontSize func-name mag)
+  "Used by @code{\\magnifyMusic} and @code{\\magnifyStaff}.  Look up the
+current @code{fontSize} in the appropriate context and scale it by the
+magnification factor @var{mag}.  @var{func-name} is either
+@code{'magnifyMusic} or @code{'magnifyStaff}."
   (make-apply-context
     (lambda (context)
-      (let* ((fontSize (ly:context-property context 'fontSize 0))
-             (new-fontSize (+ fontSize (magnification->font-size mag))))
-        (ly:context-set-property! context 'fontSize new-fontSize)))))
-
-(define-public (revert-fontSize mag)
-  "Used by @code{\\magnifyMusic}.  Calculate the previous fontSize value
-(before scaling) by factoring out the magnification factor @var{mag}."
+      (if (or (eq? func-name 'magnifyMusic)
+              ;; for \magnifyStaff, only scale the fontSize
+              ;; if staff magnification is changing
+              (staff-magnification-is-changing? context mag))
+        (let* ((where (case func-name
+                        ((magnifyMusic) context)
+                        ((magnifyStaff) (ly:context-find context 'Staff))))
+               (fontSize (ly:context-property where 'fontSize 0))
+               (new-fontSize (+ fontSize (magnification->font-size mag))))
+          (ly:context-set-property! where 'fontSize new-fontSize))))))
+
+(define-public (revert-fontSize func-name mag)
+  "Used by @code{\\magnifyMusic} and @code{\\magnifyStaff}.  Calculate
+the previous @code{fontSize} value (before scaling) by factoring out the
+magnification factor @var{mag} (if @var{func-name} is
+@code{'magnifyMusic}), or by factoring out the context property
+@code{magnifyStaffValue} (if @var{func-name} is @code{'magnifyStaff}).
+Revert the @code{fontSize} in the appropriate context accordingly.
+
+With @code{\\magnifyMusic}, the scaling is reverted after the music
+block it operates on.  @code{\\magnifyStaff} does not operate on a music
+block, so the scaling from a previous call (if there is one) is reverted
+before the new scaling takes effect."
   (make-apply-context
     (lambda (context)
-      (let* ((fontSize (ly:context-property context 'fontSize 0))
-             (old-fontSize (- fontSize (magnification->font-size mag))))
-        (ly:context-set-property! context 'fontSize old-fontSize)))))
-
-(define-public (scale-props props mag allowed-to-shrink?)
-  "Used by @code{\\magnifyMusic}.  For each prop in @var{props}, find
-the current value of the requested prop, scale it by the magnification
-factor @var{mag}, and do the equivalent of a
-@code{\\temporary@tie{}\\override} with the new value.  If
-@code{allowed-to-shrink?} is @code{#f}, don't let the new value be less
-than the current value.  Props are formatted like:
-
+      (if (or (eq? func-name 'magnifyMusic)
+              ;; for \magnifyStaff...
+              (and
+                ;; don't revert the user's fontSize choice
+                ;; the first time \magnifyStaff is called
+                (magnifyStaff-is-set? context mag)
+                ;; only revert the previous fontSize
+                ;; if staff magnification is changing
+                (staff-magnification-is-changing? context mag)))
+        (let* ((where
+                 (case func-name
+                   ((magnifyMusic) context)
+                   ((magnifyStaff) (ly:context-find context 'Staff))))
+               (old-mag
+                 (case func-name
+                   ((magnifyMusic) mag)
+                   ((magnifyStaff)
+                    (ly:context-property where 'magnifyStaffValue 1))))
+               (fontSize (ly:context-property where 'fontSize 0))
+               (old-fontSize (- fontSize (magnification->font-size old-mag))))
+          (ly:context-set-property! where 'fontSize old-fontSize))))))
+
+(define-public (scale-props func-name mag allowed-to-shrink? props)
+  "Used by @code{\\magnifyMusic} and @code{\\magnifyStaff}.  For each
+prop in @var{props}, find the current value of the requested prop, scale
+it by the magnification factor @var{mag}, and do the equivalent of a
+@code{\\temporary@tie{}\\override} with the new value in the appropriate
+context.  If @var{allowed-to-shrink?} is @code{#f}, don't let the new
+value be less than the current value.  @var{func-name} is either
+@code{'magnifyMusic} or @code{'magnifyStaff}.  The @var{props} list is
+formatted like:
 @example
-'(Slur height-limit)
+'((Stem thickness)
+  (Slur line-thickness)
+  ...)
 @end example"
   (make-apply-context
     (lambda (context)
@@ -2443,23 +2514,71 @@ than the current value.  Props are formatted like:
                (prop (cadr grob-prop-list))
                (where (if (eq? grob 'SpacingSpanner)
                         (ly:context-find context 'Score)
-                        context))
-               (grob-def (ly:context-grob-definition where grob))
-               (val (ly:assoc-get prop grob-def 1))
-               (proc (lambda (x)
-                       (if allowed-to-shrink?
-                         (* x mag)
-                         (* x (max 1 mag)))))
-               (new-val (if (number-pair? val)
-                          (cons (proc (car val))
-                                (proc (cdr val)))
-                          (proc val))))
-          (ly:context-pushpop-property where grob prop new-val)))
-      (for-each scale-prop props))))
-
+                        (case func-name
+                          ((magnifyMusic) context)
+                          ((magnifyStaff) (ly:context-find context 'Staff)))))
+               (grob-def (ly:context-grob-definition where grob)))
+          (if (eq? prop 'space-alist)
+            (let* ((space-alist (ly:assoc-get prop grob-def))
+                   (scale-spacing-tuple (lambda (x)
+                                          (cons (car x)
+                                                (cons (cadr x)
+                                                      (* mag (cddr x))))))
+                   (scaled-tuples (map scale-spacing-tuple space-alist))
+                   (new-alist (append scaled-tuples space-alist)))
+              (ly:context-pushpop-property where grob prop new-alist))
+            (let* ((val (ly:assoc-get prop grob-def 1))
+                   (proc (lambda (x)
+                           (if allowed-to-shrink?
+                             (* x mag)
+                             (* x (max 1 mag)))))
+                   (new-val (if (number-pair? val)
+                              (cons (proc (car val))
+                                    (proc (cdr val)))
+                              (proc val))))
+              (ly:context-pushpop-property where grob prop new-val)))))
+      (if (or (eq? func-name 'magnifyMusic)
+              ;; for \magnifyStaff, only scale the properties
+              ;; if staff magnification is changing
+              (staff-magnification-is-changing? context mag))
+        (for-each scale-prop props)))))
+
+(define-public (revert-props func-name mag props)
+  "Used by @code{\\magnifyMusic} and @code{\\magnifyStaff}.  Revert each
+prop in @var{props} in the appropriate context.  @var{func-name} is
+either @code{'magnifyMusic} or @code{'magnifyStaff}.  The @var{props}
+list is formatted like:
+@example
+'((Stem thickness)
+  (Slur line-thickness)
+  ...)
+@end example"
+  (make-apply-context
+    (lambda (context)
+      (define (revert-prop grob-prop-list)
+        (let* ((grob (car grob-prop-list))
+               (prop (cadr grob-prop-list))
+               (where (if (eq? grob 'SpacingSpanner)
+                        (ly:context-find context 'Score)
+                        (case func-name
+                          ((magnifyMusic) context)
+                          ((magnifyStaff) (ly:context-find context 'Staff))))))
+          (ly:context-pushpop-property where grob prop)))
+      (if (or (eq? func-name 'magnifyMusic)
+              ;; for \magnifyStaff...
+              (and
+                ;; don't revert the user's property overrides
+                ;; the first time \magnifyStaff is called
+                (magnifyStaff-is-set? context mag)
+                ;; revert the overrides from the previous \magnifyStaff,
+                ;; but only if staff magnification is changing
+                (staff-magnification-is-changing? context mag)))
+        (for-each revert-prop props)))))
+
+;; \magnifyMusic only
 (define-public (scale-beam-thickness mag)
   "Used by @code{\\magnifyMusic}.  Scaling @code{Beam.beam-thickness}
-exactly to the @var{mag} value won't work.  This uses two reference
+exactly to the @var{mag} value will not work.  This uses two reference
 values for @code{beam-thickness} to determine an acceptable value when
 scaling, then does the equivalent of a
 @code{\\temporary@tie{}\\override} with the new value."
@@ -2473,21 +2592,3 @@ scaling, then does the equivalent of a
              (scaled-default (+ 119/925 (* mag 13/37)))
              (new-val (* scaled-default ratio-to-default)))
         (ly:context-pushpop-property context 'Beam 'beam-thickness new-val)))))
-
-(define-public (revert-props props)
-  "Used by @code{\\magnifyMusic}.  Revert each prop in @var{props}.
-Props are formatted like:
-
-@example
-'(Slur height-limit)
-@end example"
-  (make-apply-context
-    (lambda (context)
-      (define (revert-prop grob-prop-list)
-        (let* ((grob (car grob-prop-list))
-               (prop (cadr grob-prop-list))
-               (where (if (eq? grob 'SpacingSpanner)
-                        (ly:context-find context 'Score)
-                        context)))
-            (ly:context-pushpop-property where grob prop)))
-      (for-each revert-prop props))))