code: function( keys ) {
  		var lookupString = keys.toString();
  		var id = this.plainObjectSignatureLookup[lookupString];			
  		if (id === undefined) {
  
  			// Create new signature buffer
  			var sigbuf = [];
  			sigbuf.push( pack2b( keys.length, false ) );
  			for (var i=0, l=keys.length; i<l; ++i) {
  				sigbuf.push( pack2b( this.stringID(keys[i]), false ) );
  			}
  
  			// Keep on table
  			this.plainObjectSignatureTable.push( Buffer.concat( sigbuf ) );
  			this.plainObjectSignatureLookup[lookupString] 
  				= id = this.plainObjectSignatureTable.length - 1;
  
  		}
  		return id;
  	}
docs: Create and/or return the property signature with the given ID