sign error in Veech group computations

There seems to be an error in Veech group computations reported by Andrea Thevis (Saarbrücken). In the example below, there is a weird -1 twist.

sage: from surface_dynamics import *
sage: o = Origami("(1,6)(2,5,4)","(1,3,2,4,5)")
sage: g = o.veech_group() 
sage: l = g.L()
sage: r = g.R()
sage: s2 = g.S2()
sage: s3 = g.S3()
sage: m = s2**2   # minus identity
sage: i = m**2    # identity

sage: m == s3**3       # OK
True
sage: (s2*l)**3 == i   # WRONG: should be m!
False
sage: (s2*r)**3 == m   # WRONG: should be i!
True