String doExt() {
		interp.getToken();
		if (interp.token!='.')
			interp.error("'.' expected");
		interp.getToken();
		if (!(interp.token==WORD || interp.token==STRING_FUNCTION || interp.token==NUMERIC_FUNCTION || interp.token==PREDEFINED_FUNCTION))
			interp.error("Function name expected: ");
		String name = interp.tokenString;
		if (name.equals("install")) {
			Object plugin = IJ.runPlugIn(getStringArg(), "");
			if (plugin==null) interp.error("Plugin not found");
			return null;
		}
		ExtensionDescriptor desc = null;
		if (pgm.extensionRegistry!=null)
			desc = (ExtensionDescriptor) pgm.extensionRegistry.get(name);
		if (desc == null) {
			interp.error("Unrecognized Ext function");
			return null;
		}
		return desc.dispatch(this);
	}