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<p>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 <b>System Label</b>.  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.</p>

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      <td valign="top"><apll>⎕PRO:</apll></td>
      <td></td>
      <td></td>
      <td>is called where a prototype element is needed as in <apll>foo¨R</apll> where <apll>R</apll> is empty, or <apll>L foo¨R</apll> where one of both of <apll>L</apll> or <apll>R</apll> is empty and the other is conformable.</td>
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      <td valign="top"><apll>⎕ID:</apll></td>
      <td></td>
      <td></td>
      <td>is called where an identity element is needed as in <apll>foo/R</apll> where <apll>R</apll> is empty.</td>
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<p>For example,</p>

<apll>&nbsp;&nbsp;&nbsp;&nbsp;∇ Z←L foo R<br />
[1]&nbsp;&nbsp;&nbsp;Z←L,R ⋄ →0<br />
[2]&nbsp;&nbsp;&nbsp;⎕PRO:Z←'Prototype' ⋄ →0<br />
[3]&nbsp;&nbsp;&nbsp;⎕ID:Z←'Identity'<br />
&nbsp;&nbsp;&nbsp;&nbsp;∇<br /></apll>

<apll>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;⎕fmt foo/⍬<br />
┌──────────┐<br />
│┌8───────┐│<br />
││Identity││<br />
│└────────┘2<br />
└∊─────────┘<br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;⎕fmt 1 foo¨⍬<br />
┌0──────────┐<br />
│┌9────────┐│<br />
││&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;││<br />
│└─────────┘2<br />
└∊──────────┘<br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;foo/¨0⍴⊂⍬<br />
NONCE ERROR<br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;foo/¨0⍴⊂⍬<br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;∧</apll>

* <p>In the Identity element case, the function is called with the reduction function's right argument prototype as the right argument to the user-defined function/operator; the left argument is undefined.  For example, in <apll>foo/3 0⍴⊂⍳4</apll>, the (right) argument passed to the function is <apll>0 0 0 0</apll>.  The return value from the function is used as the common item in the result.  Thus, the <apll>⎕ID</apll> entry point is called only once even though the result may have multiple copies of the return value.</p>

* <p>In the Prototype case, the user-defined function/operator is called with arguments that are the prototypes of the respective arguments; in the monadic case, the left argument is undefined.</p>

* <p>For the moment, if two or more system labels would be called at the same time (such as the last example above), a <apll>NONCE ERROR</apll> is signaled.</p>

* This idea was taken from the paper by David A. Rabenhorst, "[http://portal.acm.org/citation.cfm?id=801228 APL function variants and system labels]", ACM SIGAPL APL Quote Quad, APL83, Volume 13,  Issue 3  (March 1983), pp. 281-284.