Escape interpolation into entities

[mbostock]

This:

My favorite currencies are &p${"ounds"};.

should produce this:

My favorite currencies are &pounds;.

However, it instead produces:

My favorite currencies are £s;.

I think in order to do this, we’d need to know whether we’re interpolating into a possibly valid HTML entity, and if so, the ampersand needs to be escaped so that it is equivalent to:

My favorite currencies are &p${"ound"}s;.

We should also test this, which works correctly today:

My favorite currencies are ${"&pounds;"}.