input "f" -- reads and executes the commands found in the file named
f, echoing the input, printing the values, and incrementing the line number.
The file is sought in the directory containing the file currently being loaded, if any, and then along the
path, unless the name of the file begins with a slash (/).
If one of the top level expressions in the file evaluates to the symbol
end the reading of the file is stopped at that point.
If an error occurs while evaluating the expressions in the file, reading is stopped.