:: deftheorem Def1 defines + VFUNCT_1:def 1 :
for C being non empty set
for V being non empty addLoopStr
for f1, f2, b₅ being PartFunc of C,V holds
( b₅ = f1 + f2 iff ( dom b₅ = (dom f1) /\ (dom f2) & ( for c being Element of C st c in dom b₅ holds
b₅ /. c = (f1 /. c) + (f2 /. c) ) ) );