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ERC20.sol

pragma solidity ^0.7.0;

import {ERC20} from "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.2.0-solc-0.7/contracts/token/ERC20/ERC20.sol";


contract GRT is ERC20 {
    constructor ()
        ERC20 ("Graph token", "GRT")
    {
      _mint(msg.sender, 100e18);
    }
}

Staking.sol

pragma solidity ^0.7.0;

import {IERC20} from "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.2.0-solc-0.7/contracts/token/ERC20/IERC20.sol";

contract Staking {
    IERC20 token;
    mapping(address => uint256) indexerDelegation;
    
    constructor (IERC20 _token) {
      token = _token;
    }
    
     function delegate(address _indexer, uint256 _tokens)
        external
        returns (uint256)
    {
        address delegator = msg.sender;

        // Transfer tokens to delegate to this contract
        token.transferFrom(delegator, address(this), _tokens);
        indexerDelegation[_indexer] += _tokens;
        return _tokens;
    }
    function getDelegation(address _indexer) public view returns (uint256){
        return indexerDelegation[_indexer];
    }
}