This experimental option makes square brackets behave like parentheses in the context of array indices.
In FORTRAN, FTANGLE will just replace the brackets by parentheses. In C, the brackets will be left alone.
FWEAVE, however, will typeset the indices according to the fwebmac.sty macro ‘\WARRAY’. This macro takes one argument, which is just the array index or indices. (In C, indexing like ‘a[i][j][k]’ generates the argument ‘i,j,k’.) By default, ‘\WARRAY’ just surrounds its argument with brackets. However, the user may change its definition to get special effects such as superscripted or subscripted indices. A simple example macro ‘\WSUB’ is provided in fwebmac.sty; one can say ‘\let\WARRAY\WSUB’ in the limbo section to have bracketed indices print as subscripts.
This feature may not work when the contents of the brackets are too complicated (so that FWEAVE tries to typeset them by going in and out of math mode).
For more information, experts can see fwebmac.web, command \WXA.