function enableCollateral (address _collateral) external onlyGovernor {
    require(collateralExists[_collateral], "OVLV1:!exists");
    require(!collateralActive[_collateral], "OVLV1:!disabled");
    OverlayToken(ovl).grantRole(OverlayToken(ovl).MINTER_ROLE(), _collateral);
    OverlayToken(ovl).grantRole(OverlayToken(ovl).BURNER_ROLE(), _collateral);
}

function disableCollateral (address _collateral) external onlyGovernor {
    require(collateralActive[_collateral], "OVLV1:!enabled");
    OverlayToken(ovl).revokeRole(OverlayToken(ovl).MINTER_ROLE(), _collateral);
    OverlayToken(ovl).revokeRole(OverlayToken(ovl).BURNER_ROLE(), _collateral);
}
