Eval takes an expression, parse and evaluate it, everytime this method is called a new parser is created, if you want to reuse the parsed tree see the `New` method.

New create a new boolean expression parser will return an error if the expression if invalid.

Errors.

Errors.

Null is returned when the variable doesn't exist.

Expression parse a boolean expression into a tree and allow to evaluate the expression.

VarStore is the interface to implements when you want the expression engine to be able to fetch the value of a variables.