code: func If(cond ResourceConditionFunc, f schema.CustomizeDiffFunc) schema.CustomizeDiffFunc {
  	return func(d *schema.ResourceDiff, meta interface{}) error {
  		if cond(d, meta) {
  			return f(d, meta)
  		}
  		return nil
  	}
  }
docs: // If returns a CustomizeDiffFunc that calls the given condition
  // function and then calls the given CustomizeDiffFunc only if the condition
  // function returns true.
  //
  // This can be used to include conditional customizations when composing
  // customizations using All and Sequence, but should generally be used only in
  // simple scenarios. Prefer directly writing a CustomizeDiffFunc containing
  // a conditional branch if the given CustomizeDiffFunc is already a
  // locally-defined function, since this avoids obscuring the control flow.