Suppose Red and Green exchange a series of messages thus:

Date t1 Home (0, 0, 0)

Dear Green,

I hope this finds you as it leaves me.

Yours ever. Red

To which Green replies:

Date t2 Home (0, 0, 0)

Dear Red,

Thank you for your letter, dated `t1', which has just arrived.

Yours ever,

Green

Red then answers Green's Ietter:

Date t3 Home (0, 0, 0)

Dear Green,

Thank you for your letter, dated `t2', which has just arrived.

Yours ever,

Red

To which Green again replies:

Date t4 Home (0, 0, 0)

Dear Red,

Thank you for your letter, dated `t3', which has just arrived.

Yours ever,

Green

From this exchange of messages, Red knows when he sent his first message, when he received one back, and also knows the date on which Green says he, Green, received the message according to his, Green's, reckoning,

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