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Variable/Mates: for them names or marks are used - values: include all objects, which can be designated by these expressions (Convention).
no changeable things, also no names of changeable things.
Variable/free/bound/Mates: E.g. "(x)F"x": here bound for the second time - problem: simultaneously within "F"x" free. - ((S) considered without quantifier.
and formulas (if used) may occur bound.
(s) an entire formula always occurs free of course.
Elementare Logik Göttingen 1969