public int getChannelValue(String channel) {
		Integer I = channelNameToValueMap.get(channel);
		int i = (I != null) ? I : -1;
		return i;
	}
Gets the constant channel value for a user-defined channel.

<p>
This method only returns channel values for user-defined channels. All
other channels, including the predefined channels
{@link Token#DEFAULT_CHANNEL} and {@link Token#HIDDEN_CHANNEL} along with
any channel defined in code (e.g. in a {@code @members{}} block), are
ignored.</p>

@param channel The channel name.
@return The channel value, if {@code channel} is the name of a known
user-defined token channel; otherwise, -1.
tool/src/org/antlr/v4/tool/Grammar.java