void setType(ParsedType value) {
        verifyThat(type == null, "Cannot change type from ", type, "to", value);
        this.type = value;
    }
This class holds the extracted information after parsing a summary
field declaration, either from "field" inside "document-summary" or
"summary" inside "field".  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/ParsedSummaryField.java