From grammar.300:
Token sequences of the form any - (ZEI - any) ..., where there may be any number of ZEIs, are merged into a single token of selma'o BRIVLA.
Exceptional words: zo, zoi, lo'u, le'u, zei, si, sa, su ,fa'o
Anything quoted with zo, zoi or lo'u...le'u can be compounded as a whole with zei, for example: {zo da zei zoi gy something gy zei lo'u ku pai ki le'u} is a brivla.
{le'u} can be part of a zei-brivla as long as it is outside a {lo'u...le'u} quote. For example: {le'u zei le'u} is a brivla.
{zei} can be a part of a zei-brivla as long as it is not the first component. For example {da zei zei} is a brivla.
si, sa, su and fa'o are also probably not allowed in zei-brivla.