Hi,

Enclosed is a polished patch for full logical subexpressions, including 
manual updates for the syntax.  This allows the following (from the manual):

      An expression consists of search terms joined by logical
      operators -a (and) and -o (or).  The -a operator has precedence
      over -o and both are evaluated left-to-right.

      ( expr )
              True if the subexpression expr is true.

      expr1 -a expr2
              True if both expr1 and expr2 are true (logical "and").

      expr1 [-o] expr2
              True if expr1 and/or expr2 evaluate to true (logical
              "or").

      term    True if term is satisfied.  This has syntax
              [key[,key]*(=~)]?val, where operand key is an mdoc(7)
              macro to query and val is its value.  Operator =
              evaluates a substring, while ~ evaluates a regular
              expression.

      -i term
              Same as term, but term is evaluated case-insensitively.

This patch also hides away backend "struct rec" components (lhs, rhs, 
matches) to apropos_db.c and adds lots of in-line documentation.

Oks?  If so, I'll check it in and get to work merging Ingo's excellent work.

Thanks,

Kristaps