We also have the stop command which stops the execution of a procedure when it is reached. Another important control command is the if command. A predicate is an expression that evaluates to true or false. The following is a typical use of the if command in our abbreviated introduction to Logo. If the value of the variable size is less than 3, the procedure stops.
Parameters make procedures much more useful.
The length of each side is size. The three procedure given below draw a crude house. The procedure house invokes the procedures square and floor. When a procedure invokes a copy of itself, it said to be recursive. The meaning of recursive procedures is obtained in exactly the same way as regular procedural invocation, namely, via the copy rule.