code: def call(exe, *argv):
      
      exes = which(exe)
      if not exes:
          returnValue(None)
      stdout, stderr, value = yield getProcessOutputAndValue(
          exes[0], argv, env=os.environ.copy()
      )
      if value:
          returnValue(None)
      returnValue(stdout.decode('utf-8').rstrip('\n'))
docs: Run a command, returning its output, or None if it fails.