So is known to be one of the coordinating conjunctions of English. So it can introduce an independent sentence or clause. Those are defined as containing a complete thought as opposed to the incomplete thoughts expressed by dependent sentences. I have my doubts as to whether so in the following sentence is really coordinating
I did that so he could come.
Is the second part considered to be sub- or coordinated or can so be a subordinating conjunction too?
If it is considered coordinated, doesn't that hurt the anyway suboptimal definition of what subordinate really is?
I am asking because to me "So he could come" in this example does by NO means feel independent, neither grammatically nor semantically.