'This method returns the Coxeter diagram corresponding to a Weyl group.
The Coxeter diagram can be obtained from a Lie algebra\'s Dynkin diagram
by deleting all arrows; the Coxeter diagram is the undirected graph.
The vertices of the Coxeter diagram represent the generating reflections
of the Weyl group, , s_i.  An edge is drawn between s_i and s_j if the order
m(i, j) of s_i*s_j is greater than two.  If there is one edge, the order
m(i, j) is 3.  If there are two edges, the order m(i, j) is 4, and if there
are three edges, the order m(i, j) is 6.
Examples
>>> from sympy.liealgebras.weyl_group import WeylGroup
>>> c = WeylGroup("B3")
>>> print(c.coxeter_diagram())
0---0===0
1   2   3'

def coxeter_diagram(self):

    n = self.cartan_type.rank()
    if ((self.cartan_type.series == 'A') or (self.cartan_type.series == 'D') or (self.cartan_type.series == 'E')):
        return self.cartan_type.dynkin_diagram()
    if ((self.cartan_type.series == 'B') or (self.cartan_type.series == 'C')):
        diag = ('---'.join(('0' for i in range(1, n))) + '===0\n')
        diag += '            '.join((str(i) for i in range(1, (n + 1))))
        return diag
    if (self.cartan_type.series == 'F'):
        diag = '0---0===0---0\n'
        diag += '            '.join((str(i) for i in range(1, 5)))
        return diag
    if (self.cartan_type.series == 'G'):
        diag = '0\xe2\x89\xa1\xe2\x89\xa1\xe2\x89\xa10\n1            2'
        return diag
