Forcing a result from verify to return a Boolean value is too restrictive, so a more general class of objects may be returned, with the Boolean value of these being checked by the above types.

A result from a call to verify is considered to be true if the result is true.  A result from a call to verify is considered to be either false or FAIL if either of those values is returned, or if a list containing either false or FAIL as a first operand is returned, respectively.

The special verification boolean will convert all return values which are lists to return the first operand of the list.  This can be used if a boolean value is expected by some procedure.

The special verification truefalse will convert all return values which are lists to return the false.  This can be used if a truefalse value is expected by some procedure.