/*!
# `IF expression THEN statements [ELSE statements]`
Also `IF expression GOTO line`.
## Purpose
Do something contingent on a predicate.
## Remarks
Statements may omit the word GOTO.
## Example
```text
10 A=10
20 IF A<30 THEN PRINT A:GOSUB 100:GOTO 20
90 END
100 A=A+10:RETURN
RUN
10
20
```
*/