The following cmavo are discussed in this section:
fi'o FIhO modal place prefix
fe'u FEhU modal terminator
Sometimes the place structures engineered into Lojban are inadequate to meet the needs of actual speech. Consider the gismu “viska”, whose place structure is:
viska: x1 sees x2 under conditions x3
Seeing is a threefold relationship, involving an agent (le viska), an object of sight (le se viska), and an environment that makes seeing possible (le te viska). Seeing is done with one or more eyes, of course; in general, the eyes belong to the entity in the x1 place.
Suppose, however, that you are blind in one eye and are talking to someone who doesn’t know that. You might want to say, “I see you with the left eye.” There is no place in the place structure of “viska” such as “with eye x4” or the like. Lojban allows you to solve the problem by adding a new place, changing the relationship:
5.1) mi viska do fi'o kanla [fe'u] le zunle
I see you [modal] eye: the left-thing
I see you with the left eye.
The three-place relation “viska” has now acquired a fourth place specifying the eye used for seeing. The combination of the cmavo “fi'o” (of selma'o FIhO) followed by a selbri, in this case the gismu “kanla”, forms a tag which is prefixed to the sumti filling the new place, namely “le zunle”. The semantics of “fi'o kanla le zunle” is that “le zunle” fills the x1 place of “kanla”, whose place structure is
kanla: x1 is an/the eye of body x2
Thus “le zunle” is an eye. The x2 place of “kanla” is unspecified and must be inferred from the context. It is important to remember that even though “le zunle” is placed following “fi'o kanla”, semantically it belongs in the x1 place of “kanla”. The selbri may be terminated with “fe'u” (of selma'o FEhU), an elidable terminator which is rarely required unless a non-logical connective follows the tag (omitting “fe'u” in that case would make the connective affect the selbri).
The term for such an added place is a “modal place”, as distinguished from the regular numbered places. (This use of the word “modal” is specific to the Loglan Project, and does not agree with the standard uses in either logic or linguistics, but is now too entrenched to change easily.) The “fi'o” construction marking a modal place is called a “modal tag”, and the sumti which follows it a “modal sumti”; the purely Lojban terms “sumti tcita” and “seltcita sumti”, respectively, are also commonly used. Modal sumti may be placed anywhere within the bridi, in any order; they have no effect whatever on the rules for assigning unmarked bridi to numbered places, and they may not be marked with FA cmavo.
Consider Example 5.1 again. Another way to view the situation is to consider the speaker’s left eye as a tool, a tool for seeing. The relevant selbri then becomes “pilno”, whose place structure is
pilno: x1 uses x2 as a tool for purpose x3
and we can rewrite Example 5.1 as
5.2) mi viska do fi'o se pilno le zunle kanla
I see you [modal] [conversion] use: the left eye.
I see you using my left eye.
Here the selbri belonging to the modal is “se pilno”. The conversion of “pilno” is necessary in order to get the “tool” place into x1, since only x1 can be the modal sumti. The “tool user” place is the x2 of “se pilno” (because it is the x1 of “pilno”) and remains unspecified. The tag “fi'o pilno” would mean “with tool user”, leaving the tool unspecified.