More careful object/metalanguage distinction

The current fol/syn text is less than precise about when the use of a symbol is part of the object language, and when it's a part of the metalanguage, esp. when variables are concerned, e.g., a $x$ may be a schematic metavariable for variable symbols, or it may be a sloppy way of denoting a variable symbol in the language. We should clean this up; but perhaps a discussion needs to be had about how pedantic we want to be. It can cause confusion esp when we say things like "$x$ and $y$ are different variables"; "$x = y$ can be misunderstood as $s(x) = s(y)$ etc.

Start by locating the fol/syn text and reviewing its statements about object language, metalanguage, variables, and the examples involving x, y, and s(x) = s(y). Discuss how precise the terminology should be, then revise the text so the distinction and intended readings are unambiguous.