ka'o'ai zei namcu zei-lujvo

x1 is a strictly imaginary number (possibly being 0 = 0×i), not necessarily understood as a complex number as well.

Real part is 0.

On affix form:

namcu
x1 (li) is a number/quantifier/digit/value/figure (noun); refers to the value and not the symbol.
zekri
x1 (event/state) is a punishable crime/[taboo/sin] to people/culture/judges/jury x2.

In definition:

zeiljvo
x1 (text) is a Lojban compound predicate using the particle “zei”; with meaning x2, arguments x3, built from metaphor (tanru) x4.
zejvo
x1 (text) is a Lojban compound predicate using the particle “zei”; with meaning x2, arguments x3, built from metaphor (tanru) x4.
zo si si zei fa'o
x1 is a nonsense zei-lujvo using parts x2, silly by standard x3.

In notes:

ka'o zei namcu
x1 is a complex number (understood as such).