A Yorick statement can be executed or not based on the value of a scalar condition (0 means don't execute, non-0 means execute):
if (condition) statementT
or, more generally,
if (condition) statementT
else statementF
Several if statements may be chained as follows:
if (condition1) statement1
else if (condition2) statement2
else if (condition3) statement3
...
else statementF