[truly]

Almost: +13 would be rewritten to ((()))+(((13))) and -13 to ((()))-(((13))). If you interpret the empty expression as 0, perhaps it would work out.

Good question!

[tgv]

There are other unary operators (!, ~), although I'm almost sure FORTRAN didn't support those, but 1 - -13 => 1 - 0 - 13 = -12 would fail, wouldn't it?

[Taniwha]

No, not in Fortran of that era ~ was pretty much missing from most card punch keyboards