@Nullable
  private static PsiExpression findVariableDefinition(@NotNull PsiReferenceExpression referenceExpression) {
    final PsiElement resolved = referenceExpression.resolve();
    return resolved instanceof PsiVariable ? findVariableDefinition(referenceExpression, (PsiVariable)resolved) : null;
  }
@Nullable
  private static PsiExpression findVariableDefinition(@NotNull PsiReferenceExpression referenceExpression)