code: def validate_vars(env):
      
      if 'PCH' in env and env['PCH']:
          if 'PCHSTOP' not in env:
              raise SCons.Errors.UserError("The PCHSTOP construction must be defined if PCH is defined.")
          if not SCons.Util.is_String(env['PCHSTOP']):
              raise SCons.Errors.UserError("The PCHSTOP construction variable must be a string: %r"%env['PCHSTOP'])
docs: Validate the PCH and PCHSTOP construction variables.