The vegan restaurant serves 10 - 2 - 2 = <<10-2-2=6>>6 dishes with only one kind of protein.
There are 6 / 2 = <<6/2=3>>3 dishes with only beans.
There is 3 / 3 = <<3/3=1>>1 dish with only seitan.
Thus, there are 6 - 3 - 1 = <<6-3-1=2>>2 dishes with only lentils.
There are also 2 dishes with beans and lentils, so there are 2 + 2 = <<2+2=4>>4 dishes that include lentils.
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