A function returning the value -1.
10 flag=FALSE ... 100 IF answer$=correct$ flag=TRUE ... 150 IF flag PROC_got_it_right ELSE PROC_wrong
BBC BASIC (Z80) does not have true Boolean variables. Instead, numeric variables are used and their value is interpreted in a 'logical' manner. A value of 0 is interpreted as false and NOT FALSE (in other words, NOT 0 (= -1)) is interpreted as TRUE.
In practice, any value other than zero is considered TRUE. This can lead to confusion; see the keyword NOT for details.