Eager expansion of definitions’ right-hand sides can be detected

[usaoc]

For example, this program

#lang rhombus
block:
  def a:
    b
  def b = #false
  #false

makes the reference to b a syntax error, while this program

#lang rhombus
block:
  def a:
    #false
    b
  def b = #false
  #false

is a run-time exception, like the Racket counterpart.