 where the two blanks marked ‘\(F(x)\)’ are to be filled with a first-order formula having one or more free occurrences of the variable ‘\(x\)’, the blank marked ‘\(F(0)\)’ is to be filled with the same formula after each free occurrence of ‘\(x\)’ has been replaced by an occurrence of ‘0’, and the blank labeled ‘\(F(sx)\)’ is to be filled with the same formula after each free occurrence of ‘\(x\)’ has been replaced by an occurrence of ‘\(sx\)’. 