x_{1} is a real number which belongs to the interval (0, 1), or it belongs to the set x_{2} (contextless default: empty set), exactly.

This word is good for "percentage" or "fraction" (one sense). It can be used in order to construct a lujvo for "continuum" (the interval (0,1)). Generically, no further restriction is placed upon x_{1}. In particular, it need not be rational. x_{2} need not be a subset of the reals. However, in particular, x_{2 }= Set(0,1) allows x_{1} to belong to exactly [0,1]. x_{1} belongs to no set which is not a subset of (0,1) \cup x_{2}.