  Church therefore imposes the condition that the admissible place selections should be, not arbitrary functions, but effectively computable functions of the preceding outcomes in a sequence. Formally, we consider (following Wald 1938) a place selection as a function \(f\) from an initial segment \(x_1 x_2 \ldots x_{i-1}\) of a sequence \(\sigma\) into \(\{0,1\}\), such that the selected subsequence \(\sigma' = \{x_i : f(x_1 \ldots x_{i - 1}) = 1\}\), Church’s proposal is that we admit only those place selections which are computable (total recursive)  functions.[8]  Church’s proposal applies equally to the sequences von Mises is concerned with, namely those with arbitrary non-zero outcome frequencies for each type of outcome; to get the random sequences, we again make the restriction to those normal binary sequences where the limit relative frequency of each outcome is \(\frac{1}{2}\) (these are sometimes called the Church stochastic sequences).