Creep.prototype.transferFrom = function(target, resource) {
    if (target.energyCapacity && resource === RESOURCE_ENERGY) {
        return target.transferEnergy(this);
    } else if (target.storeCapacity) {
        return target.transfer()
    } else if (resource !== RESOURCE_ENERGY && target.energyCapacity) {
        return ERR_INVALID_TARGET;
    } else {
        return ERR_FULL;
    }
};