:: deftheorem DeltaL defines delta_L ROUGHIF2:def 3 :
for R being finite Approximation_Space
for b₂ being preRIF of R holds
( b₂ = delta_L R iff for x, y being Subset of R holds b₂ . (x,y) = (((CMap (kappa R)) . (x,y)) + ((CMap (kappa R)) . (y,x))) / 2 );