Extensional equality
(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 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.
