font-lock-mark-block-function is a variable defined in `font-lock.el'.
Its value is
nil
This variable may be risky if used as a file-local variable.
Documentation:
Non-nil means use this function to mark a block of text.
When called with no args it should leave point at the beginning of any
enclosing textual block and mark at the end.
This is normally set via `font-lock-defaults'.