\DOC delta_pair

\BLTYPE
delta_pair : ('a -> 'a delta) ->
             ('b -> 'b delta) ->
             'a * 'b -> ('a * 'b) delta
\ELTYPE

\SYNOPSIS
Apply two functions to the projections of a pair, sharing as much
structure as possible.

\KEYWORDS
sharing.

\DESCRIBE
An application {delta_pair f g (x,y)} applies {f} to {x} and {g} to {y}.
If {f x} equals {g y} equals {SAME}, then {SAME} is returned.
Otherwise {DIFF (p1,p2)} is returned, where {p1} is {x} if {f x}
equals {SAME}; otherwise {p1} is {f x}. Similarly, {p2} is {y} if {g y}
equals {SAME}; otherwise {p2} is {g y}.

\FAILURE
If {f x} raises {e}, then {delta_pair f g (x,y)} raises {e}.

If {g y} raises {e}, then {delta_pair f g (x,y)} raises {e}.

\EXAMPLE
See the example in the documentation for {delta_apply}.

\SEEALSO
Lib.delta, Lib.delta_apply, Lib.delta_pair.
\ENDDOC