The extern command provides you with preliminary declaration of a function or a method or a statement. The command allows you to call a function before it has been defined. For example, a recursive function call from another function.
The keyword extern is followed by the block that contains function declaration. Each line of the block contains either function or method or operator declaration, excluding their bodies.

<extern> ::= extern '{' {(<function declaration> | <method declaration> | <operator declaration> )<end-of-line>} '}'
<function declaration> ::= func [<type name>] <function name> [<function attributes>] ['(' [<parameters declaration>] ')']
<method declaration> ::= method [<type name>] <type name>'.'<method name> ['(' [<parameters declaration>] ')']
<operator declaration> ::= operator <type name> <operator> '(' <parameters declaration> ')'

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