Define a conditional IF... ELSE for a boolean-specifier  such as  VALIDITY .

opt:  Optional

1.  First, define the "attribute" of the VARIABLE

2.  Second, define the conditional statement which includes either an "IF" and a "ELSE"


We must reference a value of a different VARIABLE in this expression.  In this particular case, we are referencing temp_type as a dependent variable in the IF-expression.