function mainSafetyCheck (parsed, type = 'Literal') {
  // Check there's only one expression
  assert.strictEqual(parsed.body.length, 1)
  // Check it's what we expected
  assert.strictEqual(parsed.body[0].type, 'ExpressionStatement')
  assert.strictEqual(parsed.body[0].expression.type, type)
}
Can't be too careful with eval.
Check no-one slipped something malicious in the complex strings