THE

run-time type check 
Major Section:  PROGRAMMING

(The typ val) checks that val satisfies the type specification typ (see type-spec). An error is caused if the check fails, and otherwise, val is the value of this expression. Here are some examples. 

(the integer 3)       ; returns 3
(the (integer 0 6) 3) ; returns 3
(the (integer 0 6) 7) ; causes an error
See type-spec for a discussion of the legal type specifications.

The is defined in Common Lisp. See any Common Lisp documentation for more information.