define-hash-table-test

define-hash-table-test is a built-in function in `C source code'.

(define-hash-table-test NAME TEST HASH)

Define a new hash table test with name NAME, a symbol.

In hash tables created with NAME specified as test, use TEST to
compare keys, and HASH for computing hash codes of keys.

TEST must be a function taking two arguments and returning non-nil if
both arguments are the same. HASH must be a function taking one
argument and returning an object that is the hash code of the argument.
It should be the case that if (eq (funcall HASH x1) (funcall HASH x2))
returns nil, then (funcall TEST x1 x2) also returns nil.