A line is what is typed into the computer to be evaluated. A line may be either an expression or an equation. In the current implementation, lazy assignment is performed. The variable is bound to the expression, rather than the value of the expression. Since the variable is defined to be that expression, it is not possible to re-define the variable unless the variable is first cleared.
Assignment evaluates to the variable name.
Clear, causes all of the its arguments to become unbound. It evaluates to a list containing the variable names. It evaluates to the derivative of the expression, with respect to the variables in order. It attempts to take the integral of the expression with respect to the variable. Currently, Int is somewhat limited.