Incondicional or condicional alliance
When you give a diplomatic "share intel" or "alliance" you can give it in a Inconditional mode (+), independently whatever other player do, or in a Conditional mode (~) that you will give automatically the information if the other player also gives to you.
Also, if you are in a conditional mode (~), and the other player take out the diplomacy, you automatically remove the sharing.
player1 Incoditional (+), player2 nothing (-) => player1 shares, player2 no shares
player1 Incodicional (+), player2 conditional (~) => player1 shares, player2 shares
player1 Incodicional (+), player2 inconditional (+) => player1 shares, player2 shares
player1 condicional (~), player2 conditional (~) => player1 shares, player2 shares
player1 condicional (~), player2 noting (-) => player1 no shares, player2 no shares
player1 noting (-), player2 nothing (-) => player1 no shares, player2 no shares
and viceversa
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HARKi commented
The idea comes from Phost 4.0
http://phost.de/phost4doc/alliances.html