theorem Th1: :: RMOD_3:1

for R being Ring
for V being RightMod of R
for W1, W2 being Submodule of V
for x being object holds
( x in W1 + W2 iff ex v1, v2 being Vector of V st
( v1 in W1 & v2 in W2 & x = v1 + v2 ) )