I do not understand model 5.3 Simple Random Walk $$P[X_n=j|X_1=i_1,...,X_{m-1}=i_{m-1},X_m=i_m]$$ =$$P[X_m+Y_{m+1}+...+Y_n=j|X_1=i_1,...,X_{m-1}=i_{m-1},X_m=i_m]$$ How do you arrive at the second step
X_n is the SUM(i =1 to n ) of Y_i X_m is the SUM(i =1 to m) of Y_i So X_n = X_m + SUM( i =m+1 to n) of Y_i