I'm thinking about harmonic function in music.

If ^7 is the leading tone of the tonic, and #^4 "tonicizes" the dominant key, then #^4 is not the leading tone of the dominant key, it is the leading tone of the dominant-being-tonicized, and therefore only the leading tone of a tonic.

Does the dominant have a leading tone? Yes, as per Harrison and others, flat-^6 must be the leading tone to the dominant, which means flat-^2 is the leading tone of the tonic-being-dominantized, and therefore the "Neapolitan Sixth" chord is... dominantizing both the dominant and the tonic? Then what is the most straight-forward dominantizing chord? ^4--b^6--^1. It's the Subdominant triad.

Then how does one Subdominantize? Is this what Tonic function does?