Something

alias smith { 
  var %exp = $calc(%xp2next($4) - $exp($3)) 
  var %ironbar = $calc(%exp / 12.5) 
  var %steelbar = $calc(%exp / 17.5) 
  return c1 Iron Bars:c2 %ironbar c1 Steel Bars:c2 %steelbar 
}