AT&T 6300 Programmer's Manual page 216

COMMON
Statement
Example
7-60
10
REM
mod1
20 A=1:B=2
30 COMMON AllB
50
COMMON C
60 CHAIN
U
mod3 U
10
REM
mod2
20 A=1:B=2
30 COMMON A
40 GOTO 60
50
COMMON B
60 CHAIN
u
mo d3
u
10 REM mod3
20 PRINT AiB
For example, when executing program "modI"
an "Illegal function call in 50" is issued, as
variable C has not been initialized. When
executing program "mod2" instead, program
"mod3" is CHAINed: it displays both A and B
variables, even if statement 50 of "mod2" is
jumped over.