theorem Th6: :: PENCIL_4:6

for F being Field
for V being VectSp of F
for W1, W2, W3, W4 being Subspace of V st W1 is Subspace of W2 & W3 is Subspace of W4 & (Omega). W1 = (Omega). W3 & (Omega). W2 = (Omega). W4 holds
segment (W1,W2) = segment (W3,W4)