Normally, execution of a user-defined function/operator starts execution at line one. However, in certain contexts, execution may start at a different line depending upon the context and the presence of a System Label. These special labels start with a quad symbol so as to present no confusion with normal labels. The following table lists the system labels defined so far and the special context in which they are used.
∇ Z←L foo R
 Z←L,R ⋄ →0
 ⎕PROTOTYPE:Z←'Prototype' ⋄ →0
⎕fmt 1 foo¨⍬
Note that in the Identity element case, the function is called with the reduction function's right argument's prototype as the right argument to the user-defined function/operator; the left argument is undefined. For example, in foo/3 0⍴⊂⍳4, the (right) argument passed to the function is 0 0 0 0.