/* PL/I does not have an assert function as such, */
/* but it is something that can be implemented in */
/* any of several ways.  A straight-forward way   */
/* raises a user-defined interrupt. */

on condition (assert_failure) snap
   put skip list ('Assert failure');
....
if a ^= b then signal condition(assert_failure);

/* Another way is to use the preprocessor, thus: */
%assert: procedure (a, b) returns (character);
   return ('if ' || a || '^=' || b ||
      ' then signal condition(assert_failure);');
%end assert;
%activate assert;

assert(a, 42);