Process post-fix operators. Post-fix operators require the previous (optional) node to process. Post-fix processing only occurs while processing expressions (for example, inside a group). In particular, `\hat 2^2` will parse as `\hat{2}^2` and not `\hat{2^2}`.
(lex: &mut Lexer,
local: Style,
mut prev: Option<ParseNode>)
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