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"
EXAMPLE
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.
