@Override
    @Nullable VariableElement getElement(Elements elementUtils) {
      ExecutableElement enclosingExecutableElement =
          enclosingExecutableElementFactory.getElement(elementUtils);
      if (enclosingExecutableElement == null) {
        return null;
      } else {
        return enclosingExecutableElement.getParameters().stream()
            .filter(paramElement -> toString.equals(paramElement.toString()))
            .collect(onlyElement());
      }
    }
The index of the element among all elements of the same kind within the enclosing type. If
this is method {@code foo(...)} and the index is 0, that means that the method is the first
method called {@code foo} in the enclosing type.
common/src/main/java/com/google/auto/common/BasicAnnotationProcessor.java