compile(context, tableid, defcols) {
			//	console.log('Expression',this);
			if (this.reduced) {
				return returnTrue();
			}
			return new Function('p', 'var y;return ' + this.toJS(context, tableid, defcols));
		}
Compile AST to JavaScript expression
			@param {string} context Context string, e.g. 'p','g', or 'x'
			@param {string} tableid Default table name
			@param {object} defcols Default columns dictionary
			@return {string} JavaScript expression