Tests if a finite partial function is defined on a certain domain value.
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. The call defined f x returns true if the finite partial function f
is defined on domain value x, and false otherwise.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# defined (1 |=> 2) 1;;
val it : bool = true
# defined (1 |=> 2) 2;;
val it : bool = false
# defined undefined 1;;
val it : bool = false