Context needs to be standardized apart from new scoping variables

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If we have a context X != a and are descending an expression

Y + sum{(on X) X | X = a}

the context needs to be SA'd to X' != a so that there is no interference 
between it and the "new X".

This must be done in every descending operation, most importantly on 
Expression.replace's implementation.

Original issue reported on code.google.com by [email protected] on 8 Jun 2013 at 12:39

[GoogleCodeExporter]

Original comment by [email protected] on 8 Jun 2013 at 12:39

• Added labels: Milestone-Unscheduled