ApplicationExpression 
An application expression: (M N).

Error  
Expression 
The abstract class of a lambda calculus expression.

LambdaExpression 
A lambda expression: \x.M.

Parser 
A lambda calculus expression parser.

Set 
A collection of unique objects, with uniqueness determined by an

Variable 
A variable, either free or bound.

VariableExpression 
A variable expression which consists solely of a variable.
