Combinatoric Cheatsheet

A combinatoric cheatsheet. It include methods names and a short
description.
Modified latex printing of Lambda to use \mapsto
Modified tests so working with \mapsto version of Lambda
Removed  that I forgot about in test_latex.py
Modified tests to include parentheses around whole Lambda expression
Corrected tests for Lambda
Merge branch 'master' of https://github.com/Mba7eth/sympy into comb_cheat
Updated Makefile for combinatoric cheatsheet.
Modified func_coef
Added first method for linear system 2nd order 2eqs
Added first method for linear system 2nd order 2eqs
Added method to solve type2 of 2nd order
Resolved conflict
Modification in test cases and is_linear match
Added some methods for linear system of 2 eqs for 2nd order
Modified func_coef
Added first method for linear system 2nd order 2eqs
Added method to solve type2 of 2nd order
Modification in test cases and is_linear match
Added some methods for linear system of 2 eqs for 2nd order