*\input test
 (test.TEX 1 2 (basic.TEX 1 2 3 4)




\array #1\\#2\endarray →\vcenter {\halign {\tabskip 0pt#1\ddt \cr #2\cr }}
#1←\c \ddt \quad ⊗\c 


#2←a⊗b\\c⊗d
! You can't do that in vertical mode.
...array #1\\#2\endarray →\vcenter 
                                   {\halign {\tabskip 0pt#1\ddt \cr...
p.2,l.8 c⊗d\endarray
                    
↑i
*$\vcenter
\c →\hfill $##$\hfill 
! Missing # inserted in alignment preamble.
<to be read again> 
                   \cr 
...\halign {\tabskip 0pt#1\ddt \cr 
                                   #2\cr }}
p.2,l.8 c⊗d\endarray
                    
↑
⊗⊗⊗ restricted horizontal, spacefactor 1.0000
\glue .0000 plus 10000000000.0000
\hyphenation0
\:g a
\hyphenation1
\glue .0000 plus 10000000000.0000
⊗⊗⊗ restricted horizontal, spacefactor 1.0000
⊗⊗⊗ restricted vertical
⊗⊗⊗ restricted vertical
⊗⊗⊗ math
⊗⊗⊗ horizontal, spacefactor 1.0000
\hbox( .0000 + .0000)x 20.0000
⊗⊗⊗ vertical, prevdepth .0000
\glue .0000 plus 1.0000
⊗⊗⊗ current page:
\glue 10.0000
\hbox( .0000 + .0000)x .0000
\glue-12.0000
⊗⊗⊗ nesting level 3
! OK.
<vlist> $\hfill \ddt 
                     \quad \ENDV
<argument> a⊗
             b\\c⊗d
...align {\tabskip 0pt#1\ddt \cr #2
                                   \cr }}
p.2,l.8 c⊗d\endarray
                    
↑
\c →\hfill $##$\hfill 
! You can't do that in math mode.
\c →\hfill $##
              $\hfill 
<ulist> \c 
           \ddt 
<vlist> $\hfill \ddt \quad \ENDV
                                
<argument> a⊗
             b\\c⊗d
...align {\tabskip 0pt#1\ddt \cr #2
                                   \cr }}
p.2,l.8 c⊗d\endarray
                    
↑

⊗⊗⊗ restricted horizontal, spacefactor 1.0000
\glue .0000 plus 10000000000.0000
\hyphenation0
\hyphenation1
\glue .0000 plus 10000000000.0000
⊗⊗⊗ restricted horizontal, spacefactor 1.0000
\unsetbox( 4.4444 + .0000)x 15.2778, glueset 20000000000.0000
.\glue .0000 plus 10000000000.0000
.\hyphenation0
.\:g a
.\hyphenation1
.\glue .0000 plus 10000000000.0000
.\glue 10.0000
⊗⊗⊗ restricted vertical
⊗⊗⊗ restricted vertical
⊗⊗⊗ math
⊗⊗⊗ horizontal, spacefactor 1.0000
\hbox( .0000 + .0000)x 20.0000
⊗⊗⊗ vertical, prevdepth .0000
\glue .0000 plus 1.0000
⊗⊗⊗ current page:
\glue 10.0000
\hbox( .0000 + .0000)x .0000
\glue-12.0000
⊗⊗⊗ nesting level 3
! OK.
<ulist> \c \ddt 
                
<vlist> $\hfill \ddt \quad \ENDV
                                
<argument> a⊗
             b\\c⊗d
...align {\tabskip 0pt#1\ddt \cr #2
                                   \cr }}
p.2,l.8 c⊗d\endarray
                    

\\→\cr 
⊗⊗⊗ restricted horizontal, spacefactor 1.0000
\glue .0000 plus 10000000000.0000
\hyphenation0
\:g c
\hyphenation1
\glue .0000 plus 10000000000.0000
⊗⊗⊗ restricted horizontal, spacefactor 1.0000
⊗⊗⊗ restricted vertical, prevdepth .0000
\hbox( 6.9444 + .0000)x 20.8333
.\unsetbox( 4.4444 + .0000)x 15.2778, glueset 20000000000.0000
..\glue .0000 plus 10000000000.0000
..\hyphenation0
..\:g a
..\hyphenation1
..\glue .0000 plus 10000000000.0000
..\glue 10.0000
.\unsetbox( 6.9444 + .0000)x 5.5556, glueset 20000000000.0000
..\glue .0000 plus 10000000000.0000
..\hyphenation0
..\hyphenation1
..\glue .0000 plus 10000000000.0000
..\:a b
⊗⊗⊗ restricted vertical
⊗⊗⊗ math
⊗⊗⊗ horizontal, spacefactor 1.0000
\hbox( .0000 + .0000)x 20.0000
⊗⊗⊗ vertical, prevdepth .0000
\glue .0000 plus 1.0000
⊗⊗⊗ current page:
\glue 10.0000
\hbox( .0000 + .0000)x .0000
\glue-12.0000
⊗⊗⊗ nesting level 3
! OK.
<vlist> $\hfill \ddt 
                     \quad \ENDV
<argument> a⊗b\\c⊗
                  d
...align {\tabskip 0pt#1\ddt \cr #2
                                   \cr }}
p.2,l.8 c⊗d\endarray
                    

\c →\hfill $##$\hfill 
! You can't do that in math mode.
\c →\hfill $##
              $\hfill 
<ulist> \c 
           \ddt 
<vlist> $\hfill \ddt \quad \ENDV
                                
<argument> a⊗b\\c⊗
                  d
...align {\tabskip 0pt#1\ddt \cr #2
                                   \cr }}
p.2,l.8 c⊗d\endarray
                    

⊗⊗⊗ restricted horizontal, spacefactor 1.0000
\glue .0000 plus 10000000000.0000
\hyphenation0
\hyphenation1
\glue .0000 plus 10000000000.0000
⊗⊗⊗ restricted horizontal, spacefactor 1.0000
\unsetbox( 4.4444 + .0000)x 14.3056, glueset 20000000000.0000
.\glue .0000 plus 10000000000.0000
.\hyphenation0
.\:g c
.\hyphenation1
.\glue .0000 plus 10000000000.0000
.\glue 10.0000
⊗⊗⊗ restricted vertical, prevdepth .0000
\hbox( 6.9444 + .0000)x 20.8333
.\unsetbox( 4.4444 + .0000)x 15.2778, glueset 20000000000.0000
..\glue .0000 plus 10000000000.0000
..\hyphenation0
..\:g a
..\hyphenation1
..\glue .0000 plus 10000000000.0000
..\glue 10.0000
.\unsetbox( 6.9444 + .0000)x 5.5556, glueset 20000000000.0000
..\glue .0000 plus 10000000000.0000
..\hyphenation0
..\hyphenation1
..\glue .0000 plus 10000000000.0000
..\:a b
⊗⊗⊗ restricted vertical
⊗⊗⊗ math
⊗⊗⊗ horizontal, spacefactor 1.0000
\hbox( .0000 + .0000)x 20.0000
⊗⊗⊗ vertical, prevdepth .0000
\glue .0000 plus 1.0000
⊗⊗⊗ current page:
\glue 10.0000
\hbox( .0000 + .0000)x .0000
\glue-12.0000
⊗⊗⊗ nesting level 3
! OK.
<ulist> \c \ddt 
                
<vlist> $\hfill \ddt \quad \ENDV
                                
<argument> a⊗b\\c⊗
                  d
...align {\tabskip 0pt#1\ddt \cr #2
                                   \cr }}
p.2,l.8 c⊗d\endarray
                    
)
! Missing $ inserted.
<to be read again> 
                   \par 
(*) \input test
                

*$\par\vfill\end
! Missing $ inserted.
<to be read again> 
                   \par 
(*) $\par
         \vfill\end

Completed for page 1:
\vbox( 480.0000 + .0000)x 353.0000, glueset .0000
.\glue 10.0000
.\hbox( .0000 + .0000)x .0000
.\glue-12.0000
.\glue .0000 plus 1.0000
.\glue .0278
.\hbox( 11.9722 + 6.9722)x 353.0000, glueset .0031
..\hbox( .0000 + .0000)x 20.0000
..\hyphenation0
..\vbox( 18.9444 + .0000)x 20.8333, shifted 6.9722
...\hbox( 6.9444 + .0000)x 20.8333
....\glue .0000
....\hbox( 4.4444 + .0000)x 15.2778
.....\glue .0000 plus 10000000000.0000
.....\hyphenation0
.....\:g a
.....\hyphenation1
.....\glue .0000 plus 10000000000.0000
.....\glue 10.0000
....\glue .0000
....\hbox( 6.9444 + .0000)x 5.5556
.....\glue .0000 plus 10000000000.0000
.....\hyphenation0
.....\hyphenation1
.....\glue .0000 plus 10000000000.0000
.....\:a b
....\glue .0000
...\glue 5.0556
...\hbox( 6.9444 + .0000)x 20.8333
....\glue .0000
....\hbox( 4.4444 + .0000)x 15.2778, glueset .0000
.....\glue .0000 plus 10000000000.0000
.....\hyphenation0
.....\:g c
.....\hyphenation1
.....\glue .0000 plus 10000000000.0000
.....\glue 10.0000
....\glue .0000
....\hbox( 6.9444 + .0000)x 5.5556
.....\glue .0000 plus 10000000000.0000
.....\hyphenation0
.....\hyphenation1
.....\glue .0000 plus 10000000000.0000
.....\:a d
....\glue .0000
..\hyphenation1
..\glue .0000 plus 100000.0000
.\glue .0000 plus 1.0000
.\glue 5.0278
.\hbox( .0000 + .0000)x 353.0000, glueset .0033
..\hbox( .0000 + .0000)x 20.0000
..\hyphenation0
..\hyphenation1
..\glue .0000 plus 100000.0000
.\glue .0000 plus 10000000000.0000
\ctrline #1→\hbox to size{\hskip 0pt plus1000cm minus1000cm #1\hskip 0pt plus1000cm minus1000cm}
#1←\:a\count 0 [1]