test_imag = ->
  run_test [

    "imag(a+i*b)",
    "b",

    "imag(1+exp(i*pi/3))",
    "1/2*3^(1/2)",

    "imag(i)",
    "1",

    "imag((-1)^(1/3))",
    "1/2*3^(1/2)",

    "imag(-i)",
    "-1",
  ]