public Severity atLeast(final Severity minimumSeverity) {
        if (minimumSeverity == null || this.greaterThanOrEqual(minimumSeverity)) {
            return this;
        } else {
            return minimumSeverity;
        }
    }
Return this severity if it is greater than or equal to minimumSeverity
else return minimumSeverity.
stroom-util-shared/src/main/java/stroom/util/shared/Severity.java