change ERC20Token to interface

contracts/token/ERC20Token.sol

@@ -1,48 +1,40 @@
-pragma solidity ^0.4.17;
+pragma solidity ^0.4.23;
 
 // Abstract contract for the full ERC 20 Token standard
 // https://github.com/ethereum/EIPs/issues/20
 
-contract ERC20Token {
-    /* This is a slight change to the ERC20 base standard.
-    function totalSupply() constant returns (uint256 supply);
-    is replaced with:
-    uint256 public totalSupply;
-    This automatically creates a getter function for the totalSupply.
-    This is moved to the base contract since public getter functions are not
-    currently recognised as an implementation of the matching abstract
-    function by the compiler.
-    */
-    /// total amount of tokens
-    uint256 public totalSupply;
-
+interface ERC20Token {
+  
     /// @notice send `_value` token to `_to` from `msg.sender`
     /// @param _to The address of the recipient
     /// @param _value The amount of token to be transferred
     /// @return Whether the transfer was successful or not
-    function transfer(address _to, uint256 _value) public returns (bool success);
+    function transfer(address _to, uint256 _value) external returns (bool success);
 
     /// @notice `msg.sender` approves `_spender` to spend `_value` tokens
     /// @param _spender The address of the account able to transfer the tokens
     /// @param _value The amount of tokens to be approved for transfer
     /// @return Whether the approval was successful or not
-    function approve(address _spender, uint256 _value) public returns (bool success);
+    function approve(address _spender, uint256 _value) external returns (bool success);
 
     /// @notice send `_value` token to `_to` from `_from` on the condition it is approved by `_from`
     /// @param _from The address of the sender
     /// @param _to The address of the recipient
     /// @param _value The amount of token to be transferred
     /// @return Whether the transfer was successful or not
-    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
+    function transferFrom(address _from, address _to, uint256 _value) external returns (bool success);
 
     /// @param _owner The address from which the balance will be retrieved
     /// @return The balance
-    function balanceOf(address _owner) public view returns (uint256 balance);
+    function balanceOf(address _owner) external view returns (uint256 balance);
 
     /// @param _owner The address of the account owning tokens
     /// @param _spender The address of the account able to transfer the tokens
     /// @return Amount of remaining tokens allowed to spent
-    function allowance(address _owner, address _spender) public view returns (uint256 remaining);
+    function allowance(address _owner, address _spender) external view returns (uint256 remaining);
+
+    /// total amount of tokens
+    function totalSupply() external view returns (uint256 supply);
 
     event Transfer(address indexed _from, address indexed _to, uint256 _value);
     event Approval(address indexed _owner, address indexed _spender, uint256 _value);

contracts/token/StandardToken.sol

@@ -4,23 +4,24 @@ import "./ERC20Token.sol";
 
 contract StandardToken is ERC20Token {
 
+    uint256 private supply;
     mapping (address => uint256) balances;
     mapping (address => mapping (address => uint256)) allowed;
 
     constructor() internal { }
-
+    
     function transfer(
         address _to,
         uint256 _value
     ) 
-        public 
+        external 
         returns (bool success)
     {
         return transfer(msg.sender, _to, _value);    
     }
 
     function approve(address _spender, uint256 _value) 
-        public
+        external
         returns (bool success) 
     {
         allowed[msg.sender][_spender] = _value;
@@ -33,7 +34,7 @@ contract StandardToken is ERC20Token {
         address _to,
         uint256 _value
     )
-        public
+        external
         returns (bool success)
     {
         if (balances[_from] >= _value &&
@@ -47,7 +48,7 @@ contract StandardToken is ERC20Token {
     }
 
     function allowance(address _owner, address _spender) 
-        public 
+        external 
         view 
         returns (uint256 remaining)
     {
@@ -55,12 +56,31 @@ contract StandardToken is ERC20Token {
     }
 
     function balanceOf(address _owner) 
-        public 
+        external 
         view 
         returns (uint256 balance) 
     {
         return balances[_owner];
     }
+    
+    function totalSupply() 
+        external 
+        view 
+        returns(uint256 supply) 
+    {
+        return supply;
+    }
+
+    function mint(
+        address _to,
+        uint256 _amount
+    ) 
+        internal
+    {
+        balances[_to] += _amount;
+        supply += _amount;
+        emit Transfer(0x0, _to, _amount);
+    }
 
     function transfer(
         address _from,