Mutually Recursive Data-Structures in Ahis on csnet a couple of times a

[Mats_Ohlin_FOA2%QZCOM.MAILNET@MIT-MULTICS.ARPA]

This is how it works (on a DEC-10 system):
.;---------------------------
.;create module 1
.make a1.sim
*ioptions(/E);          -- tells the compiler that this is an ext. module
class a1;
begin  ref (b1) bx;  end a1;
$ex$$

.comp a1.sim           -- compile module 1
SIMULA:  A1
begin  ref (b1) bx;  end a1;
SIM331  E LINE     3     QUALIFICATION b1 IS NOT A CLASS  -- b1 is unknown

.; create module 2, now including external decl.
.make b1.sim
*ioptions(/E);
external class a1;      -- now OK since A1.ATR exists, however uncomplete
class b1;
begin  ref (a1) ax;  end b1;
$ex$$

.dir/n ?1
A1      SIM     1  <057>   20-Sep-85    A1      ATR     1  <057>   20-Sep-85
A1      REL     1  <057>   20-Sep-85    B1      SIM     1  <057>   20-Sep-85

.com b1.sim            -- compile module 2
SIMULA:  B1

EXIT

.te a1.sim
*liexternal class b1;           -- add external decl.
$ht$$
options(/E);
external class b1;
class a1;
begin  ref (b1) bx;  end a1;
*ex$$

.com a1.sim            -- recompile module 1 using correct B1.ATR
SIMULA:  A1

NEW ATR FILE GENERATED          -- compiler warns user
RECOMPILE DEPENDENT PROGRAMS    -- that recompilation may be necessary

EXIT

.com/com b1.sim
SIMULA:  B1

NEW ATR FILE GENERATED

EXIT

.dir/n ?1
A1      BAK     1  <057>   20-Sep-85    A1      QTR     1  <057>   20-Sep-85
A1      REL     1  <057>   20-Sep-85    B1      SIM     1  <057>   20-Sep-85
B1      QTR     1  <057>   20-Sep-85    B1      REL     1  <057>   20-Sep-85
A1      SIM     1  <057>   20-Sep-85    A1      ATR     1  <057>   20-Sep-85
B1      ATR     1  <057>   20-Sep-85

.;--- QTR files are backup versions of created ATR files
.;--- *very* useful whenever you find that you would rather
.;--- backtrack than force everyone (dependent) to recompile...