private void verifySource(String source, SummaryField summaryField, DocumentSummary summary) {
        if ( ! isValid(source, summaryField, summary) ) {
            throw new IllegalArgumentException("For " + schema + ", document-summary '" +
                                               summary.name() + "'," + " summary field '" + summaryField.getName() +
                                               "': there is no valid source '" + source + "'.");
        }
    }
Verifies that the source fields actually refers to a valid field.

@author baldersheim
config-model/src/main/java/com/yahoo/schema/processing/SummaryFieldsMustHaveValidSource.java