code: def check_pre_requirements(pre_requirements):
      
      pre_requirements = set(pre_requirements or [])
      pre_requirements.add('virtualenv')
  
      for requirement in pre_requirements:
          if not which(requirement):
              print_error('Requirement {0!r} is not found in system'.
                          format(requirement))
              return False
  
      return True
docs: Check all necessary system requirements to exist.
  
      :param pre_requirements:
          Sequence of pre-requirements to check by running
          ``where <pre_requirement>`` on Windows and ``which ...`` elsewhere.