Sympad

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* Added translation from messy SymPy representation of symmetric set difference to internal simple one.
* Changed test_sym to work better and test more cases.
* Fixed many small grammatical inconsistencies.
* Fixed matrix simplification error during integration for ODE solver.

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* Added differentiation now also parses and writes out forms dy/dx and df/dx.
* Added parsing and representation of numerical subscripts.
* Modified differential recognition in integral parsing to work better - \int dx/x, \int *lambda* dx, etc...
* Fixed erroneous mapping of bound variables in lim, sum, diff and intg.

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* Added variables tab to browser client.
* Added lambdas as expression in proper contexts (can add, multiply, differentiate, etc...).
* Added S(num) is automatically converted to just num.
* Changed lambda function parsing to be more robust and raised lambda keyword binding precedence.

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* Added definition of lambdas via assignment to undefined function - f(x) = x**2.
* Modified undefined functions to be specifiable implicitly without '?' in most contexts.

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* Added undefined function initial conditions - ?y(x)(0) = something, ?y(x)'(0) = something
* Added implicit undefined functions - y(x) -> ?y(x)

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* Added undefined functions (for ODEs).
* Added prime ' differentiation operator.
* Added semicolon separator to allow multiple expressions on single line.
* Cleaned up internal negative object parsing and printing rules.
* Fixed internal lambda variable representation so lambda defined variables doesn't get remapped.