Applies a finite partial function, failing on undefined points.
DESCRIPTION
This is one of a suite of operations on finite partial functions, type
('a,'b)func. These may sometimes be preferable to ordinary functions since
they permit more operations such as equality comparison, extraction of domain
etc. If f is a finite partial function and x an argument, apply f x tries
to apply f to x and fails if it is undefined.
EXAMPLE
# apply undefined 1;;
Exception: Failure "apply".
# apply (1 |=> 2) 1;;
val it : int = 2