All treatments of so-called to-contraction in English (Lakoff 1970, Bresnan 1971, Lightfoot 1976) attempt to deal with the phenomenon entirely in terms of contiguity. The purpose of this squib is to point out that the major constraint on this phenomenon is rather one of subject coreference, and furthermore that this coreference condition is identical to that for a universal rule which I have called 'equi-subject clause union' (Frantz 1976).
Frantz, Donald G.
"A new view of to-contraction,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 21, Article 2.
Available at: https://commons.und.edu/sil-work-papers/vol21/iss1/2