(Or extensionality). Functions, f and g are extensionally equal if and only if f x = g x for all x. where "=" means both expressions fail to terminate (under some given href="module.php?name=Lexikon&file=search&eid=1&query=reduction strategy">reduction strategy) or they both terminate with the same basic value. Two functions may be extensionally equal but not inter-convertible (neither is reducible to the other). E.g. x . x+x and x . 2*x. See also {observational equivalence}, {referential transparency}.
