cl-assert is an autoloaded Lisp macro in `cl-macs.el'.

(cl-assert FORM &optional SHOW-ARGS STRING &rest ARGS)

Verify that FORM returns non-nil; signal an error if not.
Second arg SHOW-ARGS means to include arguments of FORM in message.
Other args STRING and ARGS... are arguments to be passed to `error'.
They are not evaluated unless the assertion fails. If STRING is
omitted, a default message listing FORM itself is used.