It's basically a switch statement, but without the benefit of descriptive case identifiers. So it's basically,
switch (I-20) {
case 21:
case 22:
...
}
but you have to imagine "21" as the string identifier, not the integer value. `I-20` is indexing the list of labels in the GOTO statement.
The earlier column beginning with
GOTO (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15) I
is easier to read
switch (I) {
case 1:
...
case 2:
...
}
because the integer value of I, the string labels, and their index in the list are all the same.
I've never written Fortran. I did learn to program using TI-85 BASIC, though, and am well versed in C. The GOTO in the Fortran code is possibly the easiest part for me to understand.
If you look at the code, they seem to follow a similar convention. But the computed GOTO described above is using a set of labels employed for what looks like an inner state machine, related to a previous inner state machine (thus the offset by 20). It seems perfectly reasonable (preferable, even) to use an entirely different domain of numeric labels, distinguished by length, for those parts of the code.
EDIT- Terrifying:
251 GOTO (21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37) I-20