Message4997

Assignment of a newly created ast Node value fails.  The assignment
destination become 'None' rather then accepting the value.

Test case:
Simple code mutation.  Trying manipulate the ast tree to change a
methods call like
a.set( 1 )
to the assignment:
a = 1

By running attached code 'astMutationTest.py' ('unparse.py' code is
obtained from Cpython parse demo code)

the python output is:
Before Mutation

a.set(1)

After Mutation

a = 1


But for Jython:
Before Mutation

a.set(1)

After Mutation

None