+** The file is formatted as a sequence of lines, each taken
+** atomically. The first character of each line describes how
+** the line is to be interpreted. The lines are:
+** Dxval Define macro x to have value val.
+** Cxword Put word into class x.
+** Fxfile [fmt] Read file for lines to put into
+** class x. Use scanf string 'fmt'
+** or "%s" if not present. Fmt should
+** only produce one string-valued result.
+** Hname: value Define header with field-name 'name'
+** and value as specified; this will be
+** macro expanded immediately before
+** use.
+** Sn Use rewriting set n.
+** Rlhs rhs Rewrite addresses that match lhs to
+** be rhs.
+** Mn p f r a Define mailer. n - internal name,
+** p - pathname, f - flags, r - rewriting
+** rule for sender, a - argument vector.
+**