:: deftheorem Def18 defines -boundary POLYFORM:def 18 :
for p being polyhedron
for k being Integer
for b₃ being Function of (k -chain-space p),((k - 1) -chain-space p) holds
( b₃ = k -boundary p iff for c being Element of (k -chain-space p) holds b₃ . c = Boundary c );