ady,
let us exploit them to introduce some of the notation and terminology
that we'll be needing later on.  The syntacto-graphic entity e_3 that
we used a while ago is not really a member of the club L_2, but a bit
more like a presiding pattern whose precedents in L_2 are e_1 and e_2.
What it really is, is nothing but a name for the set {e_1, e_2} c L_2.
If you find yourself tempted to dub such a thing a &quot;metaformula&quot;, let
me strongly counsel you to resist that occlusion of syntax, as I find
that 