sgmlpre is a program that handle SGML conditionalization for SGML-tools
It is used by other programs in sgml-tools (v1), and usually normal user does not need to use this program directly by himself.
Following is quoted from the header in the source code.
Filter SGML according to conditionalizing markup. Argument/value pairs from the command line are matched against the attributes of <#if> and <#unless> tags. Spans between <#if>/</#if> are passed through unaltered if there is no attribute mismatch; spans between <#unless></#unless> are passed if there is at least one attribute mismatch. An attribute mismatch happens if an attribute occurs in both the command-line arguments and the tag, but the values do not match. Value matching is by string equality, except that "|" is interpreted as an alternation character. Even if a span is not passed through, its newlines are (this to avoid messing up the line numbers in error messages).
This lexer requires flex. Limitations; attribute names may only be 256 chars long, values may be only 16384 (YY_BUF_SIZE) characters long. Doesn't do checking that only </if> matches <if> and </unless> matches <unless> (that would need a stack and introduce another limit).
This manual page was written by Taketoshi Sano <email@example.com>, for the Debian GNU/Linux system (but may be used by others).