x1 is (n)th member of alphabet/set x2 ordered by rule x3, where the count begins at x4.
Technically, if moi2 is specified as a particular alphabet, then that word is equivalent to this one modulo moi'u4; however, when no explicit and particular specification is given, "moi" can refer to a far larger set of possibilities whereas this word is meant specifically to encode the symbols in a character set/alphabet to numbers. Typically, n will be restricted to nonnegative integers; however, it may occasionally be useful to pretend that there were, for example, a fictitious letter halfway between "a" and "b" called "a.5" so to speak, so numbers which are not nonnegative integers are definitionally possible in principle for n; similarly, for n < x4 or n > |x2| + x_4 - 1, the output may be defined in any number of ways (such as by looping or beginning a new alphabet/extending x2) or may not be defined/throw an error. An alphabet is any set of symbols (including the empty set) and is defined so as to include the terminology adopted in mathematics. Some alphabets may not normally/traditionally be laid out in one-dimensional order (rather, it may fall into multidimensional tables); in such cases, n may be an ordered tuple which indicates a particular symbol in the alphabet. When n = x4, the output is the first (so to say) letter/element in the alphabet under the relevant ordering x3. The symbols in the alphabet need not be lerfu and might even represent numbers (this is the distinction between the symbol/digit "4" and the number 4; the former represents but is not the latter).