u=x - y
The symbol - is used as infix notation for the Lie subtraction. The elements $x$ and $y$ must hade the same weight and sign.
i1 : L = lieAlgebra{a,b} o1 = L o1 : LieAlgebra
i2 : b b b a - a a a b o2 = (a a b a) + (b b b a) o2 : L