public static FieldPathEntry newStructFieldEntry(Field fieldRef) {
        return new FieldPathEntry(fieldRef);
    }
Creates a new field path entry that references a struct field.
For these kinds of field path entries, getFieldRef() is valid.

@param fieldRef The field to look up in the struct.
@return The new field path entry
document/src/main/java/com/yahoo/document/FieldPathEntry.java