x1 (text / virtual-text-object) is a variable representing x2
goi is a special case that makes, e.g. zo ko'a, a goilka'i of something. However the main use of this brivla is to allow ko'a itself, or something like la .strin., to be a virtual "text" standing in for some object, while la'e is used to extract the referent from it. This allows propositions to be truth-conditional on what a variable represents at that point of time, rather than conditional on the referent itself. See sumka'i, goilgau, goilbi'o, krati.