+.SS Special Handling
+A few functions have different behavior depending on what command they are
+being invoked from.
+.PP
+In
+.BR repl
+the (\fIformataddr\fR\^) function stores all email addresses encountered into
+an internal cache and will use this cache to suppress duplicate addresses.
+If you need to create an address list that includes previously-seen
+addresses you may use the (\fIconcataddr\fR\^) function, which is identical
+to (\fIformataddr\fR\^) in all other respects. Note that (\fIconcataddr\fR\^)
+will NOT add addresses to the duplicate-suppression cache.
+.SS Other Hints and Tips
+Sometimes to format function writers it is confusing as to why output is
+duplicated. The general rule to remember is simple: If a function or
+component escape is used where it starts with a %, then it will generate
+text in the output file. Otherwise, it will not.
+.PP
+A good example is a simple attempt to generate a To: header based on
+the From: and Reply-To: headers:
+.PP
+.RS 5
+.nf
+%(formataddr %<{reply-to}%|%{from})%(putaddr To: )
+.fi
+.RE
+.PP
+Unfortuantely if the Reply-to: header is NOT present, the output line that is
+generated will be something like:
+.PP
+.RS 5
+.nf
+My From User <from@example.com>To: My From User <from@example.com>
+.fi
+.RE
+.PP
+What went wrong? When performing the test for the
+.B if
+clause (%<), the component is not output because it is considered an
+argument to the
+.B if
+statement (hence the rule about the lack of % applies). But the component
+escape in our
+.B else
+statement (everything after the `%|') is NOT an argument to anything; the
+syntax is that it is written with a %, and thus the value of that component
+is output. This also has the side effect of setting the
+.I str
+register, which is later picked up by the (\fIformataddr\fR\^) function
+and then output by (\fIputaddr\fR\^). This format string has another bug
+as well; there should always be a valid width value in the
+.I num
+register when (\fIputaddr\fR\^) is called, otherwise bad formatting can take
+place.
+.PP
+The solution is to use the (\fIvoid\fR\^) function; this will prevent the
+function or component from outputting any text. With this in place (and
+using (\fIwidth\fR\^) to set the
+.I num
+register for the width, a better implementation would look like:
+.PP
+.RS 3
+.nf
+%(formataddr %<{reply-to}%|%(void{from})%(void(width))%(putaddr To: )
+.fi
+.RE
+.PP
+It should be noted here that the side-effects of functions and component
+escapes still are in force: as a result each component
+test in the
+.B if\-elseif\-else\-endif
+clause sets the
+.I str
+register.
+.PP
+As an additional note, the (\fIformataddr\fR\^) and (\fIconcataddr\fR\^)
+functions have some behavior when it comes to the
+.I str
+register. The starting point of the register is saved and is used to
+build up entries in the address list.
+.PP
+You will find the
+.B ap
+and
+.B fmtdump
+utilities invaluable in debugging problems with format strings.