Indexed field assignment on parse trees

[PieterOlivier]

You can access parts of a parse tree using indexing, but you cannot assign subtrees using the same method:

module bug::IndexedFieldAssignment

syntax S = A "$";
syntax A = "a" | "A";

import ParseTree;
import IO;

test bool testIndexedFieldAssignment() {
    S s = parse(#S, "a$");
    value a = s[0];
    assert "<a>" == "a";

    s[0] = (A)`A`;
    return "<s>" == "A$";
}

The assertion succeeds but the test fails with an error:

Unsupported subscript of type int on type S
Advice: |https://www.rascal-mpl.org/docs/Rascal/Errors/CompileTimeErrors/UnsupportedSubscript|

For the sake of consistency and usefulness of this feature, I think indexed assignment should be supported.