/usr/share/swi-prolog/pack/clpBNR/prolog/clpBNR.pl
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 midpoint(?X:numeric, ?M:number) is semidet
Succeeds if X is numeric and M unifies with the midpoint of X; otherwise fails. Examples: 
?- X:: real(1r2,5r3), midpoint(X,M).
M = 13r12,
X::real(0.5, 1.6666666666666667).

?- midpoint(42,M).
M = 42.

midpoint is also available as an arithmetic function:

?- X::real(1r2,pi), M is midpoint(X).
M = 1.8207963267948968,
X::real(0.5, 3.1415926535897936).