In one aspect, a device (100) includes: a display (110) configured to display symbols; a processor (112); and a computer-readable medium having stored thereon program instructions, that when executed by the processor (112), cause the device (100) to perform a set of acts including: selecting a first symbol-set from a global symbol-group, wherein the first symbol-set includes at least two symbols; displaying on the display (110) the selected first symbol-set; making a determination that a second symbol-set, which is a proper sub-set of the selected first symbol-set, satisfies a trigger condition; after making the determination, (i) selecting a first supplementary-symbol from a first supplementary symbol-group different from the global symbol-group and (ii) selecting a second supplementary-symbol from a second supplementary symbol-group different from the global symbol-group; displaying on the display (110) the selected first supplementary-symbol; displaying on the display (110) the selected second supplementary-symbol; determining a first amount based, at least in part, on the selected first symbol-set; determining a second amount based, at least in part, on the selected first supplementary-symbol and the selected second supplementary-symbol; determining a payout amount based on a function of the determined first amount and the determined second amount; and displaying on the display (110) the determined payout amount.