em(Identifiers) are names that can have a special meaning in the YODL
language. E.g., the sequence tt(INCLUDEFILE) is an identifier: when
followed by a filename in parentheses, YODL will take some special action (in
this case, read the file as a YODL-source file).
    
Identifiers may consist of uppercase or lowercase characters. No other
characters may appear in them. 

In particular, em(note) that this diverts from the well known definition for
identifiers used in most programming languages: identifiers may not contain
underscores, nor digits. YODL, therefore, won't accept identifiers like
tt(run4) or tt(under_score).
    
em(Names) are sequences of characters, not containing white space characters.
(i.e., any series of characters not containing spaces, tabs or
newlines). Names are allowed with certain builtin functions, like the
tt(INCLUDEFILE) function, expecting the name of a file as its argument.
    
