ModuleType[] allowed(Position position) {
		ModuleType[] result = allowed[position.ordinal()];
		if (result == null) {
			throw new IllegalArgumentException(String.format("A %s can't have a module at position '%s'",
					this.name(), position.name()));
		}
		return result;
	}
Return the kinds of modules that may appear at some position in the current parsing context.
spring-xd-dirt/src/main/java/org/springframework/xd/dirt/stream/ParsingContext.java