fact: Definition psum f : R := let S := [set x | exists J : {fset T}, x = \sum_(x : J) `|f (val x)| ]%classic in if `[<summable f>] then sup S else 0.
imports: mathcomp(all_ssreflect all_algebra) mathcomp.classical(boolp) xfinmap ereal reals discrete realseq mathcomp.classical(classical_sets functions mathcomp_extra) topology