def p_formula_atom(self, p):
        """formula : ATOM
                   | TRUE
                   | FALSE"""
        if p[1]==Symbols.TRUE.value:
            p[0] = PLTrue()
        elif p[1]==Symbols.FALSE.value:
            p[0] = PLFalse()
        else:
            p[0] = PLAtomic(Symbol(p[1]))