* climbing up the syntax tree, passing control back and forth between the
     * two.
     *
     * Whether an expression is used is defined by the context in which it
     * appears. E.g. a "statement" in a block is considered used if it is the
     * last expression in that block AND the block itself is used -- a
     * recursive call to `isUsed`, one level higher in the syntax tree.
     *