'The ith simple root of G_2
Every lie algebra has a unique root system.
Given a root system Q, there is a subset of the
roots such that an element of Q is called a
simple root if it cannot be written as the sum
of two elements in Q.  If we let D denote the
set of simple roots, then it is clear that every
element of Q can be written as a linear combination
of elements of D with all coefficients non-negative.
Examples
>>> from sympy.liealgebras.cartan_type import CartanType
>>> c = CartanType("G2")
>>> c.simple_root(1)
[0, 1, -1]'

def simple_root(self, i):

    if (i == 1):
        return [0, 1, (-1)]
    else:
        return [1, (-2), 1]
