types

PPD :: S: Stock
       P: Pricelist
       E: EquipDb;

Stock = map Unit to Quantity;
Unit = Equip | Comp;
Quantity = nat;
Equip :: K: nat;
Comp  :: K: nat;
Pricelist = map Comp to Currency;
Currency = real;
EquipDb = map Equip to map Unit to Quantity