docs: function _filesToStream (files, separator, callback) {
  
  			if ("undefined" === typeof files) {
  				throw new ReferenceError("missing \"files\" argument");
  			}
  				else if ("object" !== typeof files || !(files instanceof Array)) {
  					throw new TypeError("\"files\" argument is not an Array");
  				}
  			else if ("undefined" === typeof callback && "undefined" === typeof separator) {
  				throw new ReferenceError("missing \"callback\" argument");
  			}
  				else if ("function" !== typeof callback && "function" !== typeof separator) {
  					throw new TypeError("\"callback\" argument is not a function");
  				}
  			else {
  
  				const writeStream = new Transform();
  
  				writeStream._transform = (chunk, enc, cb) => {
  					writeStream.push(chunk);
  					cb();
  				};
  
  				writeStream.once("error", () => {
  					writeStream.end();
  				});
  
  				const _callback = "undefined" === typeof callback ? separator : callback;
  				_callback(null, writeStream);
  
  				_readContent(files, writeStream, "string" === typeof separator ? separator : " ").then(() => {
  					writeStream.end();
  					writeStream.emit("close");
  				}).catch((err) => {
  					writeStream.emit("error", err);
  				});
  
  			}
  
  		}