Support comparing only n-th matching token

[qpanda]

At the moment all matching tokens are compared against the value specified in conditions of expressions. It would be useful to be able to specify that only the n-th matching token (e.g. the 3rd integer on a line) should be compared with the value specified in a condition.
The syntax for this could be an array index expression [n] added to the variable (e.g. $integer[3] == 8).