'The ith simple root of F_4
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("F4")
>>> c.simple_root(3)
[0, 0, 0, 1]'

def simple_root(self, i):

    if (i < 3):
        return basic_root((i - 1), i)
    if (i == 3):
        root = ([0] * 4)
        root[3] = 1
        return root
    if (i == 4):
        root = ([Rational((-1), 2)] * 4)
        return root
