http://lilypond.org/doc/v2.13/Documentation/usage/command_002dline-usage#environment-variables
Maybe in **this sentence** something is missing?
"LILYPOND_GC_YIELD
With this variable the memory footprint and performance can be
adjusted. **It is a percentage tunes memory management behavior**. With
higher values, the program uses more memory, with smaller values, it
uses more CPU time. The default value is 70.
"
I mean: 'tunes' should be a verb, right?
"The percentage tunes memory management...", if I've understood
correctly.
So it could be: "It is a percentage *that* tunes memory management
behaviour".
I don't know if this is good english, but I can understand it better.