Question is as stated. Where is most-positive-fixnum defined? lisp.h?
In src/data.c
DEFVAR_LISP ("most-positive-fixnum", Vmost_positive_fixnum,
doc: /* The largest value that is representable in a Lisp integer. */);
Vmost_positive_fixnum = make_number (MOST_POSITIVE_FIXNUM);
XSYMBOL (intern_c_string ("most-positive-fixnum"))->constant = 1;