+/*
+ Output settings for a block of music.
+
+ This devolved into a rather empty class. The distinction between
+ various instances is made in the parser, which creates
+ midi/layout/paper blocks depending on the keyword read.
+
+ The data structure is set up as recursive: the definitions not
+ supplied in layout are looked up in paper. This is done through
+ the parent_ field of Output_def. However, such nesting is limited to
+ two levels,
+
+ * first because the parser hard-codes the different types
+ of output block.
+
+ * Second, because the prime benefit of multiple levels
+ (eg. paper containing layout for a score, containing layout of a
+ \score embedded in \markup) requires scaling the Stencils (eg. the
+ one coming from score at markup level)
+