substring and search

[Stephen Berman]

Evalling the following expression:

(substring "abcdefg" (progn (beginning-of-line) (search-forward "c")))

yields the error "Args out of range: "abcdefg", 16, 7" (assuming the
expression is on the first line of a buffer).  Why is this?  And what
does "7" in the error message refer to?  It appears no matter where
the above sexp is evalled, e.g.:

(substring "abcdefg" (progn (beginning-of-line) (search-forward "c")))
=> Args out of range: "abcdefg", 431, 7

(search-backward also yields a corresponding error message, as do the
re-search functions.)

Thanks,
--Steve Berman