@Override
    public boolean equals(final Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        final FieldIndex that = (FieldIndex) o;
        return fieldToPos.equals(that.fieldToPos);
    }
@return True if fieldName matches the special Event ID field.
stroom-query/stroom-query-language/src/main/java/stroom/query/language/functions/FieldIndex.java