theorem :: RUSUB_2:44

for V being RealUnitarySpace
for W1, W2 being Subspace of V holds
( W1 + W2 = UNITSTR(# the carrier of V, the ZeroF of V, the U7 of V, the Mult of V, the scalar of V #) iff for v being VECTOR of V ex v1, v2 being VECTOR of V st
( v1 in W1 & v2 in W2 & v = v1 + v2 ) ) by Lm13;