Substitution changes the multiplicative unit, F(1), in F = FreeGroup(2) to
the integer, 1. The following code demonstrates the problem:
---------------------------------------------
F = FreeGroup(2)
F.inject_variables()
u = F(1)
w = u.subs({x0:x1})
print 'u.parent() = ', u.parent()
print 'w.parent() = ', w.parent()
-----------------------------------------------
Both u and w print as '1', but the parent of u is 'Free Group on generators
{x0, x1}', and the parent of w is 'Integer Ring'.
Consequently, u*x0 == x0 works fine as expected, but w*x0 == x0 results in:
TypeError: unsupported operand parent(s) for *: 'Integer Ring' and
'Free Group on generators {x0, x1}'
I am running Sage version 8.1 (sage-8.1-OSX_10.12.6-x86_64) on a MacBook
Pro, with OS X, version 10.13.3.
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