Why not positional variation?

[Koen G]

Furthermore, Schwerdtfeger has described in 2008 the following four design rules for the VMS (see You are not allowed to view links. Register or Login to view. and You are not allowed to view links. Register or Login to view.): 
(1) line-glyphs can follow line-glyphs or [a]; 
(2) curve-glyphs and [a] can follow curve-glyphs; 
(3) the [l]-glyph can be used as a curve-glyph or as a line-glyph; and 
(4) gallows glyphs count as curve glyphs.

I like this, it's a very brief and elegant way of describing the system. I find the case of a especially intriguing. Is it like the glue between curve glyphs and line glyphs? Is it simply the shape the first minim takes? (In that case, there would always be at least two minims/line-glyphs in a sequence). Is it a e - i ligature, as Patrick mentions? That would imply that there is a free-floating curve available whenever lines will follow.