docs: 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]))