Issue 4133: Give predicate ly:grob-properties? the description "grob properties"
\set Staff.InstrumentName = "Test"
previously led to
warning: type check for `InstrumentName' failed;
value `"Test"' must be of type `ly:grob-properties'
and will now instead produce
warning: type check for `InstrumentName' failed;
value `"Test"' must be of type `grob properties'.
The output looks slightly less cryptic and more intentional. The plural
form is indeed correct: this type of context property serves as a
container for _all_ grob properties of InstrumentName.