increment counter-name indexer? (by numeric-expression)?

increment increases the value of a counter by a specified amount. If no amount is specified, increment increases the value of the counter by 1:

  local counter fred initial {10}
  ;fred = 10
  increment fred
  ;fred = 11
  increment fred by 3
  ;fred = 14

decrement decreases the value of a counter by a specified amount. If no amount is specified, decrement decreases the value of the counter by 1.

  local counter fred initial {10}
  ;fred = 10
  decrement fred
  ;fred = 9
  decrement fred by 3
  ;fred = 6