Does anyone know why the space after the row-join operator and before the tempname matrix matters in the following?

Code:
tempname tn
mata m=J(1,3,2)
mata `tn'=m
mata m \ m
mata m\m
mata `tn' \ `tn'
mata `tn'\(`tn')
mata `tn'\`tn'
Code:
. tempname tn

. mata m=J(1,3,2)

. mata `tn'=m

. mata m \ m
       1   2   3
    +-------------+
  1 |  2   2   2  |
  2 |  2   2   2  |
    +-------------+

. mata m\m
       1   2   3
    +-------------+
  1 |  2   2   2  |
  2 |  2   2   2  |
    +-------------+

. mata `tn' \ `tn'
       1   2   3
    +-------------+
  1 |  2   2   2  |
  2 |  2   2   2  |
    +-------------+

. mata `tn'\(`tn')
       1   2   3
    +-------------+
  1 |  2   2   2  |
  2 |  2   2   2  |
    +-------------+

. mata `tn'\`tn'
invalid expression
r(3000);

end of do-file

r(3000);