(VAR x y )
(RULES 
        lcs(e, y) -> 0
        lcs(x, e) -> 0
        lcs(a(x), a(y)) -> s(lcs(x, y))
        lcs(b(x), b(y)) -> s(lcs(x, y))
        lcs(a(x), b(y)) -> max(lcs(x, b(y)), lcs(a(x), y))
        lcs(b(x), a(y)) -> max(lcs(x, a(y)), lcs(b(x), y))
        max(x, 0) -> 0
        max(0, y) -> 0
        max(s(x), s(y)) -> max(x, y)
        
)