Problem running mclf on Sage 8.7

[AndrewVSutherland]

After cloning mac_lane and MCLF, I tried to execute the example in the README.md and ran into the following error while running Sage 8.7 on line 168 of superp.pyc in the call to FunctionField, which has a comment noting that it does not work in Sage 8.0.

I understand that this problem should go away in Sage 8.8, which I will try, but I'm reporting the issue as suggested by Stefan so that we have a record of it.

sage: K = QQ
sage: v_3 = pAdicValuation(K, 3)
sage: R.<x> = K[]
sage: Y = Superp(x^4+1, v_3, 3)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-27-bbb2bc42f1d0> in <module>()
----> 1 Y = Superp(x**Integer(4)+Integer(1), v_3, Integer(3))

/home/drew/github/MCLF_/mclf/semistable_reduction/superp.pyc in __init__(self, f, vK, p)
    166         self._a = a
    167         self._vK = vK
--> 168         FX = FunctionField(vK.domain(), names=R.variable_names())  # this does not work in Sage 8.0
    169         S = PolynomialRing(FX, 'T')
    170         T = S.gen()

/scratch/sage/sage-8.7/local/lib/python2.7/site-packages/sage/structure/factory.pyx in sage.structure.factory.UniqueFactory.__call__ (build/cythonized/sage/structure/factory.c:2138)()
    367         key, kwds = self.create_key_and_extra_args(*args, **kwds)
    368         version = self.get_version(sage_version)
--> 369         return self.get_object(version, key, kwds)
    370 
    371     cpdef get_object(self, version, key, extra_args):

/scratch/sage/sage-8.7/local/lib/python2.7/site-packages/sage/structure/factory.pyx in sage.structure.factory.UniqueFactory.get_object (build/cythonized/sage/structure/factory.c:2529)()
    410         except KeyError:
    411             pass
--> 412         obj = self.create_object(version, key, **extra_args)
    413         self._cache[version, cache_key] = obj
    414         try:

/scratch/sage/sage-8.7/local/lib/python2.7/site-packages/sage/rings/function_field/constructor.pyc in create_object(self, version, key, **extra_args)
    105         else:
    106             from .function_field import RationalFunctionField
--> 107             return RationalFunctionField(key[0],names=key[1])
    108 
    109 FunctionField=FunctionFieldFactory("sage.rings.function_field.constructor.FunctionField")

mac_lane/__init__.pyc in __init__(self, *args, **kwargs)

/scratch/sage/sage-8.7/local/lib/python2.7/site-packages/sage/structure/parent.pyx in sage.structure.parent.Parent.register_conversion (build/cythonized/sage/structure/parent.c:14678)()
   1658         self._action_list.append(action)
   1659 
-> 1660     cpdef register_conversion(self, mor):
   1661         r"""
   1662         Update the coercion model to use `\text{mor} : P \to \text{self}` to convert

/scratch/sage/sage-8.7/local/lib/python2.7/site-packages/sage/structure/parent.pyx in sage.structure.parent.Parent.register_conversion (build/cythonized/sage/structure/parent.c:14398)()
   1678             TypeError: ...
   1679         """
-> 1680         assert not (self._coercions_used and mor.domain() in self._convert_from_hash), "conversion from %s to %s already registered or discovered"%(mor.domain(), self)
   1681         if isinstance(mor, map.Map):
   1682             if mor.codomain() is not self:

AssertionError: conversion from Rational function field in x over Rational Field to Univariate Polynomial Ring in x over Rational Field already registered or discovered