When playing Chemtech Enforcer (which requires discarding a card to play), can an opponent flip their hidden card in response to playing the Enforcer or its discard cost?

Ruling: The opponent cannot react to the discard cost itself, but can react to Chemtech Enforcer's triggered ability that triggers after it enters the board.

Sequence:

• Chemtech Enforcer is played and its discard cost is paid
• Enforcer enters the board
• Enforcer's triggered ability triggers
• Opponent can now react to that triggered ability (including flipping hidden cards)