Verify equivalent logic for conditional statements

[brynrhodes]

The following rewrite of a function in QICoreCommon:

define function ToPrevalenceInterval(condition Condition):
if condition.clinicalStatus ~ "active"
  or condition.clinicalStatus ~ "recurrence"
  or condition.clinicalStatus ~ "relapse" then
  Interval[start of ToInterval(condition.onset), end of ToAbatementInterval(condition)]
else if (end of ToAbatementInterval(condition) is null) 
   then Interval[start of ToInterval(condition.onset), null)
   else Interval[start of ToInterval(condition.onset), end of ToAbatementInterval(condition)] 

resulted in different behavior than the equivalent formulation with a let:

define function ToPrevalenceInterval(condition Condition):
if condition.clinicalStatus ~ "active"
  or condition.clinicalStatus ~ "recurrence"
  or condition.clinicalStatus ~ "relapse" then
  Interval[start of ToInterval(condition.onset), end of ToAbatementInterval(condition)]
else
  (end of ToAbatementInterval(condition)) abatementDate
    return if abatementDate is null then
      Interval[start of ToInterval(condition.onset), abatementDate)
    else
      Interval[start of ToInterval(condition.onset), abatementDate]

This is related to the following CQLIT ticket: https://oncprojectracking.healthit.gov/support/browse/CQLIT-426

In theory, the above functions should behave exactly the same. Create a unit test to verify this for all the edges involved:

Condition.clinicalStatus as active and inactive Condition.onset as a dateTime (null and non-null Condition.onset as an Interval (null and non-null start and end dates) Condition.abatement as a dateTime (null and non-null) Condition.abatement as an Interval (null and non-null start and end dates)