What does the triple question mark mean in scala? -

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i can't seem find anywhere. example have method such 

def somefunction(a: string): string ={        ??? }

what triple question marks signify?

??? defined in predef , throws notimplementederror:

def ??? : nothing = throw new notimplementederror

it has return type of nothing sub-type of every type , can therefore used in place of return value.


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