function trade(uint256 amount) public {
	// some code
	// you can pass the address of the deployed SkillsCoin contract as a parameter to the constructor of the RareCoin contract as 'source'
	(bool ok, bytes memory result) = source.call(abi.encodeWithSignature("transferFrom(address,address,uint256)", msg.sender, address(this), amount));
	// this will fail if there is insufficient approval or balance
	require(ok, "call failed");
	// more code
}