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ST-20 token is an Ethereum-based token implemented on top of the ERC-20 protocol that adds the ability for tokens to control transfers based on specific rules.
Its description: https://github.com/PolymathNetwork/polymath-core.
Look at how there are implemented ERC-20 and ERC-721 in https://github.com/matterinc/web3swift/tree/develop/web3swift/PrecompiledContracts.