field projecten of complex lexical syntax types (nested sequences) does not work in the interpreter

[jurgenvinju]

Describe the bug

This is a lexical definition with a head field and a tail field.

lexical X = ([a-z] head [a-z]+ tail)+ words;

Instances of those trees should allow for field projection and field update like so:

X x = ...; if (y <- x.words) println(y.head);

But they don't. The reason is that the interpreter assumes all non-terminal types to be context-free and thus it skips layout positions which aren't there in a lexical tree (of a sequence combinator).