clpBNR:delta/2

Succeeds if X is numeric and W unifies with the width of X (upper bound-lowerbound); otherwise fails. Examples:

?- X:: real(1r2,5r3),delta(X,D).
D = 7r6,
X::real(0.5, 1.6666666666666667).

?- delta(42,W).
W = 0.

delta is also available as an arithmetic function:

?- X::real(1r2,pi), W is delta(X).
W = 2.6415926535897936,
X::real(0.5, 3.1415926535897936).