The next kata is the extend the previous day's binary option one period model to a multiperiod one.
For example two period's equations will look something like this in pseudo code
C_0 = math.exp(-R * (T/2)) * (P * C_U + (1 - P) * C_D)
C_D looks like
C_D = math.exp(-R * (T/2)) * (P * C_DU + (1 - P) * C_DD)
C_U = math.exp(-R * (T/2)) * (P * C_UU + (1 - P) * C_UD)
In this case,
C_DD are all known possible terminal values, just as
C_D were in the previous kata.
If we needed to we could extend to as many periods as needed with the same pattern.
When we shuffle the above together,
C_0 = math.exp( -R * (T/2) ) * ( P^2 * C_UU + P * (1-P) * C_UD + (1 - P)^2 * C_DD + (1-P) * P * C_DU )
The exercise today, is to recursively generate these binomial trees for any number of periods, just using lambdas.