Return ith calling argument.

pick(i, r_0, r_1, ...) returns r_i. Thus, if i==0, pick() returns the
second calling argument (r_0); if i==1, pick() returns the third calling argument (r_1); and so on. If there is no argument corresponding to i, pick() returns an empty matrix. If pick() is called with fewer than two calling arguments, pick() throws an error.

For example, the following returns z= x if flag equals 0 or false and z= y if flag equals 1 or true:

z= pick(flag, x, y);