This loads a NESL file into the current environment. If present, the expression exp must evaluate to a string (sequence of characters) and be a valid filename. If the filename ends with the suffix ``.nesl'', the suffix can be omitted. If exp is omitted, it defaults to the last file that was loaded, or edited using edit (see Section 4.2). Files are loaded relative to the current directory. NESL files can contain any NESL top-level expressions or top-level commands; a file can therefore load other files, or set various environment variables, such as the memory size (see Section 3.5).