Refactor expressions:
Refactor expressions:

* use var i in for loop
* don't make another copy when removing term from expression

timrach/algebra.js,mariotristan/algebra.js,VeraWei/algebra.js,mattknox/algebra.js,shaunstanislaus/algebra.js,AlexisArce/algebra.js,kevinbarabash/algebra.js,cesarmarinhorj/algebra.js,Jobava/algebra.js,HugoRLopes/algebra.js,johnRedden/algebra.js,nicolewhite/algebra.js,mubassirhayat/algebra.js,Drooids/algebra.js,kwangkim/algebra.js,kelsadita/algebra.js