symbolic L,M;

I := {[j,i] : 1 <= j <= 100 && j-L <= i <= j};

D := I*I intersection {[j1,i1] -> [j2,i2] : i1+j1 = i2+M};
I;
D;
range D;
(range D) - (range D);
E := I - range D;
sample := {[-M+3,-M+3]: L = 0 && -97 <= M <= 2};
sample intersection range D;
sample intersection E;