Work on integration with python counterpart

evm/interfaces/IPairFunctions.sol

@@ -15,7 +15,7 @@ import "interfaces/IPairFunctionsTypes.sol";
 /// @dev Last but not least, the price function is the derivative of the swap
 /// @dev function. It represents the best possible price a user can get from a
 /// @dev pair after swapping x of the specified token.
-/// @dev During calls to price, swap and getLimits, the caller can be assumed to
+/// @dev During calls to swap and getLimits, the caller can be assumed to
 /// @dev have the required sell or buy token balance as well as unlimited approvals
 /// @dev to this contract.
 interface IPairFunctions is IPairFunctionTypes {
@@ -46,7 +46,11 @@ interface IPairFunctions is IPairFunctionTypes {
     /// @dev the swap and change the state of the evm accordingly.
     /// @dev Please include a gas usage estimate for each amount. This can be achieved
     /// @dev e.g. by using the `gasleft()` function.
-    /// @dev
+    /// @dev The return type trade, has a price attribute which should contain the
+    ///      value of `price(specifiedAmount)`. As this is optional, defined via
+    ///      `Capability.PriceFunction`, it is valid to return a zero value for this
+    ///      price in that case it will be estimated numerically. To return zero use
+    ///      Fraction(0, 1).
     /// @param pairId The ID of the trading pair.
     /// @param sellToken The token being sold.
     /// @param buyToken The token being bought.
@@ -79,10 +83,6 @@ interface IPairFunctions is IPairFunctionTypes {
         external
         returns (Capabilities[] memory);
 
-    /// @notice Minimum gas usage of exchange logic excluding transfers.
-    /// @return gasUsage the amount of gas used by the exchange logic
-    function minGasUsage() external view returns (uint256);
-
     /// @notice Retrieves the tokens in the selected pair.
     /// @dev Mainly used for testing as this is redundant with the required substreams
     /// @dev implementation.

evm/interfaces/IPairFunctionsTypes.sol

@@ -12,9 +12,9 @@ interface IPairFunctionTypes {
     }
 
     /// @dev The Capabilities enum represents possible features of a trading pair.
-    enum Capabilities
-    // Support SwapSide.Sell values (required)
-    {
+    enum Capabilities {
+        Unset,
+        // Support SwapSide.Sell values (required)
         SellSide,
         // Support SwapSide.Buy values (optional)
         BuySide,