Calculate rolling summation or subtraction between parent and child fields

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This script retrieves values from the Expense field in the issue subtasks, and then subtracts them from the budget.

RollingFieldSummationParentChild.sil script

string [] stasks = subtasks(key);
int budget = customfield_10714;

if(isNotNull(budget)) {
    for(string s in stasks) {
        if(isNotNull(%s%.customfield_10713)) {
            budget = budget - s.customfield_10713;
        }
    }
}

return budget;