neutral start: responder accepts every offer with probability 1/2
     step  E[offer]  modal offer  proposer payoff   acceptance prob. of offers 0..6
      100     1.012            1            8.665   0.50 0.97 0.72 0.72 0.72 0.72 0.73
     1000     1.001            1            8.979   0.50 1.00 0.73 0.73 0.73 0.73 0.73
    10000     1.000            1            8.998   0.50 1.00 0.74 0.73 0.73 0.74 0.74
   100000     1.000            1            9.000   0.50 1.00 0.74 0.74 0.74 0.74 0.75
  1000000     1.000            1            9.000   0.50 1.00 0.75 0.75 0.75 0.75 0.75
  5000000     1.000            1            9.000   0.50 1.00 0.75 0.75 0.75 0.75 0.76

fair-minded start: responder accepts offer x with probability sigmoid(2 (x - 4))
     step  E[offer]  modal offer  proposer payoff   acceptance prob. of offers 0..6
      100     5.004            5            4.954   0.00 0.00 0.02 0.14 0.74 1.00 0.98
     1000     5.000            5            4.996   0.00 0.00 0.02 0.14 0.78 1.00 0.99
    10000     5.000            5            5.000   0.00 0.00 0.02 0.14 0.81 1.00 0.99
   100000     5.000            5            5.000   0.00 0.00 0.02 0.15 0.83 1.00 0.99
  1000000     5.000            5            5.000   0.00 0.00 0.02 0.15 0.86 1.00 0.99
  5000000     5.000            5            5.000   0.00 0.00 0.02 0.15 0.88 1.00 0.99
