public boolean isPresent(final @NonNull String flag) {
        final List value = this.flagValues.get(flag);
        return value != null && !value.isEmpty();
    }
Check whether a presence flag is present. This will return {@code false}
for all value flags.

@param flag Flag name
@return {@code true} if the flag is a presence flag and is present,
        else {@code false}
cloud-core/src/main/java/org/incendo/cloud/parser/flag/FlagContext.java