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The readonly indexing function is used to create a table for which assigning new entries is illegal. Any object can be initially stored in a table that uses the readonly indexing function, but trying to into the table will raise an exception.

$\mathrm{table}\left('\mathrm{readonly}'\,\left[\right]\right)$

$table\left(\mathrm{readonly}\,\left[\right]\right)$

$\mathrm{table}\left('\mathrm{readonly}'\,\left[a=1\,b=2\right]\right)$

Error, (in `index/readonly`) cannot assign to a readonly table

$T≔\mathrm{subsop}\left(1='\mathrm{readonly}'\,\mathrm{table}\left(\left[a=1\,b=2\right]\right)\right)$

$T≔table\left(\mathrm{readonly}\,\left[a=1\,b=2\right]\right)$

$T\left[a\right]$

$1$

$T\left[c\right]≔1$

Error, (in `index/readonly`) cannot assign to a readonly table

$T\left[a\right]≔1$

Error, (in `index/readonly`) cannot assign to a readonly table