u=x + y
The symbol + is used as infix notation for the Lie addition. The elements $x$ and $y$ must have 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