public void addField(ParsedField field) {
        String fieldName = field.name().toLowerCase();
        verifyThat(! docFields.containsKey(fieldName),
                   "Duplicate (case insensitively) " + field + " in document type '" + this.name() + "'");
        docFields.put(fieldName, field);
    }
This class holds the extracted information after parsing a
"document" block in a schema (.sd) file, using simple data
structures as far as possible.  Do not put advanced logic here!
@author arnej27959
config-model/src/main/java/com/yahoo/schema/parser/ParsedDocument.java