World change: variables (F, B or S) change their reference world (i.e.,
External, Interpreted or Expected); e.g., this occurs in the focussing steps
from Fi to Fei (Figure 1, process 7), from Bi to Bei (Figure 1, process 8)
and from Si to Sei (Figure 1, process 9);
Class change: variables (F, B, or S) are produced from the variables that
belong to another class but still refer to the same world (i.e., External, Interpreted
or Expected), e.g., this occurs in the transformation steps from
Si to Bi (Figure 1, process 14) or from Bei to Fei (Figure 1, process 10).