AMG: ?: is the ternary operator, so called because it takes three arguments.
condition ? true_value : false_value
It evaluates to true_value if condition is true. Otherwise, it evaluates to false_value.
Unlike most expr operations, this one does not have a command counterpart in the tcl::mathop namespace -- mostly because if command already provides the would-be functionality (if returns the value returned by the branch it evaluates).