Exclamation mark in a syntax table

[Óscar Fuentes]

On a mode for an internal programming language that considers a leading
`!' as *not* part of the symbol, I put this:

      (modify-syntax-entry ?! "'   " table)

but symbol-at-point for !foo still returns !foo. Furthermore, I see no
way of specifying that a leading ! is an expression prefix but in any
other place it is a symbol constituent.