INTERFACE

(interface
   (defun function1 (a b c))
   (defun function2 (d e))
   (defun function3 ()))

The interface body contains functions but the body of the function is ignored for this instruction.

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As the actual details of how an interface is actually implemented can vary greatly from language to language it is better to refer to the language specific guidelines.

As JavaScript doesn't really have "interfaces" (that's not to say they aren't do-able using syntactic juggling with classes) the only currently supported language back-end that renders this instruction is PHP. To that end pleaser refer to the Coding for PHP Guidelines pages for further information.