c-at-expression-start-p is a compiled Lisp function in `
Return non-nil if the point is at the first token in an expression or
statement, or somewhere in the syntactic whitespace before it.
An "expression" here is a bit different from the normal language
grammar sense: It's any sequence of expression tokens except commas,
unless they are enclosed inside parentheses of some kind. Also, an
expression never continues past an enclosing parenthesis, but it might
contain parenthesis pairs of any sort except braces.
Since expressions never cross statement boundaries, this function also
recognizes statement beginnings, just like `c-at-statement-start-p'.
Note that this function might do hidden buffer changes. See the
comment at the start of cc-engine.el for more info.