This word does not mean just i, unlike what the current definition in most languages says here.
Zeroth, the CLL description gives it two properties: "?ka'o? is both a special number (meaning ?i?) and a number punctuation mark (separating the real and the imaginary parts of a complex number).". So, if we are going by official documentation, both properties should be mentioned in the definitions here.
But I have two problems even with that:
First, this is at best overloading and at worst mathbreaking. It is fine that it can act as a digit, but it cannot function as both a separator and as a representation of the unit i, which is a number unto itself. Either this is shorthand for "_ + _i" (sorta as "gei" is shorthand for "_*(_^_)", roughly, and denoted usually as "_E_"; the analogy breaks because they are of different selma'o), or it means simply "i"; it cannot be both.
Just as importantly, in some base representations (confer quaterimaginary and then generalize a bit), the differences between a pure digit "i" which means i (and which would use up one digit slot in a string) and the 'punctuation mark' meaning described by the CLL are incompatible and would cause breakage and confusion/syntactic and semantic ambiguities. Here is a simple example: Suppose that we expand traditional unbalanced decimal notation so that "i" is now a digit which works like any other? (such as "3"); then the string "1i2" ("pa ka'o re") means 1*10^2 + i*10^1 + 2*10^0 = 102 + 10i, which is distinct from the 'punctuation mark' interpretation which would result in the meaning 1 + 2i; even more strangely, "1ii2" ("pa ka'o ka'o re") would be allowed, meaning 1002 + 110i, but it is not at all clear that this would be even meaningful in the 'punctuation mark' interpretation (regardless of considerations about its actual meaning).
Second, I am not even sure that these two interpretations are grammatically compatible. The first treats "ka'o" like "ci" (they both are PA); the second treats it something like "su'u" (basically, it is a VUhU masquerading as a PA; see previous discussion about "gei"). Now, it is up for debate, but some standards of Lojban may assume implicit "xo'ei"'s when VUhU slots are left open. If this is the case, then "ka'o" alone cannot generally mean just i = 0 + 1i; it must mean x + yi, and this x can easily be nonzero and the y can easily be nonone. So, in such standards, the two interpretations conflict unresolvably. Moreover, we have already hit on two other grammatical problems here. The first is that this word, according to the CLL, is essentially a fusion of PA and VUhU, which are disjoint selma'o, which means that such a blurring is prohibited for the sake of syntactic disambiguity. The second is the case before of "1ii2" being grammatically allowed but essentially (gramamtically, not just semantically!) uninterpretable according to the current description. The first part must be the real part and the second part must be the imaginary part. Not only are these implied to be restricted to (extended) real domains, but now there are multiple real parts and multiple imaginary parts, and multiple reading orders of these. What would "1i2i3" mean? How can that even be interpreted according to the rules, grammatically? Does the first "i" or the second "i" come first? In the former case, how can it be that "1i2" the real part of the second one?
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My solution:
We introduce a new word which is really a digit meaning "i" of selma'o PA. It works just like "ci". On its own (not in a nontrivial string of digits) it means i. If it is in a nontrivial string of digits, then it is interpreted according to the representational base. So, just as "20" can mean many things (consider the bases: decimal, hexadecimal, phinary, quaterimaginary, balanced ternary, and even binary (where it is improper)), "2i" can mean many things, each dependent on the base of interpretation (specified by "ju'u"; the meaning comes from the baseinterpretation step in the Order of Interpretation). It is rather unlikely, in any event, that "2i" means 2*i, though. Just as "21" does not mean 2*1 = 2 in most bases, "2i" will not mean 2*i in most bases. With such a word, the multiplication operator must be explicitly used between the two singledigit numbers 2 and the unit i (which is, again, represented by the digit "i" on its own) in order to mean 2*i. There would be no "slot" on either side of "i" in this scenario; a string of digits could of course be multiple digits long (such as "1i2i3ii4i"), but there would be no inherent structure to the digit "i" (which would implicitly be filled by "xo'ei"'s in some Lojban systems). Complicated strings, such as the one just mentioned, would be interpreted according to the base. For this specific purpose, I propose "ka'o'ai". This word is exactly like "ci" but means i instead of 3.
Meanwhile, I simultaneously propose that we redefine/clarify/restructure/reduce the meaning of "ka'o" so that it joins selma'o VUhU (only) and that it is a binary mekso operator (like "gei", but binary) which means exactly "X1 + (X2*i)" such that X1 and X2 (the first and second argument respectively) are (as a contextless default) typically understood/restricted to be (strictly) real numbers (and this is their meaning when this is used in a veljvo unless explicitly contradicted). Thus, its arguments are not even necessarily a real numbers (but if both are, then the first is the real part of the result and the second is the imaginary part of the result). It could be denoted by "(X1, X2)" in C. Thus, it would follow whatsoever is the reading/priority order applied to mekso operators according to the culture and the utterances prior to its use. It would then have slots to be filled and would thus, according to some systems of Lojban, take on implicit "xo'ei"'s whensoever those were not explicitly filled. In some ways, this would be a macro for an expression involving "su'u", "pi'i", and a lone "ka'o'ai" (as described in the immediately previous paragraph), and nothing else (except operator prioritizers). For example, "pa ka'o re ka'o ka'o ci" could mean (if we just read lefttoright/firsttolast for priority) (((1,2), x), 3) = (((1 + 2i) + x*i) + 3i) = 1 + (5+x)i, where x is represented by an implicit "xo'ei" and could mean any (probably complex, perhaps real, possibly nonzero) number. Other reading orders would produce different (perhaps more exotic) results.
In my personal usages, I use these words to mean exactly these things (and I believe that implicit "xo'ei"'s are beneficial and maximize exotic utility, such as in my aforementioned examples). Thus, in my "zei"lujvo involving "ka'o", I mean the operator with implicit "xo'ei"'s as zero or more of its operands (even in the veljvo of these constructs).
Note, by the way, that if we ignore my proposals, then "ka'o" can be interpreted basically as my "ka'o'ai" would be. In this case, "ka'o zei namcu" would mean "x1 is a strictly imaginary number (including 0), not necessarily understood as a complex number as well". To me, this is the definition of "ka'o'ai zei namcu". Notice its similarity with the meaning of "real number" which you knew prior to being introduced to the imaginary unit and complex numbers  a consideration of R which does not account for C. To me, "ka'o zei namcu" means and can mean only "x1 is a complex number" (where the real and imaginary parts (really, the first and second operands of "ka'o") are implicit "xo'ei"'s).
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? The only problem with this notational system is that there will need to be countably infinitely many separate digits, one for each iZ (where "Z" denotes the set of all integers); presently, there are only two: "0" and "i" (which means 1*i). But this representational base is selected for demonstration purposes only.
