Sum -- the class of all sum expressions

Functions and methods returning a sum expression :

• "Expression + Expression" -- see Expression -- the class of all expressions
• "Expression - Expression" -- see Expression -- the class of all expressions

Methods that use a sum expression :

• "substitute(Sum,Thing)" -- see substitute -- substituting values for variables

For the programmer

The object Sum is a wrapper type, with ancestor classes AssociativeExpression < Expression < BasicList < Thing.