(Or extensionality).Functions, f and g are extensionally equal if and only iff x = g xfor 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 andx . 2*x.See also {observational equivalence}, {referential transparency}.