code: function _shellEscape (params, options = {}) {
      let escaped = [ __shellEscape(params, options) ]
      let flattened = _.flattenDeep(escaped)
      return flattened.join(' ')
  }
docs: /*
  performs the following mappings on the supplied value(s):
  
  - already-escaped/preserved values are passed through verbatim
  - null and undefined are ignored (i.e. mapped to empty arrays,
  which are pruned by flatten)
  - non-strings are stringified e.g. false -> "false"
  
  then flattens the resulting array and returns its elements joined by a space