Input/Output

$\Sigma{}^{{\rm it}}$ inherits the stream I/O model provided by SALLI. In particular SumitType inherits OutputType from SALLI, which means that elements of a SumitType can be written in text format to any TextWriter (in particular I/O streams, strings or files). Because it is desirable to output mathematical objects such as matrices and polynomials in more than one format depending on the context, $\Sigma{}^{{\rm it}}$ types do not in general define their own $<<$ function. Rather, they implement the extree function, that converts their elements into elements of ExpressionTree. Expression trees, wich are similar to Lisp's S-expressions, are the unique layer between all the types in $\Sigma{}^{{\rm it}}$ and the outside world. When writing your own types, once you provide or inherit an implementation of extree, your elements can then be written to any TextWriter in any of the many formats that ExpressionTree understands, for example using lisp to produce Lisp output. In that case, the default behavior of the $<<$ is to convert your objects to expression trees and then to use tex to produce TEXoutput. You can override this default behaviour by providing your own implementation of $<<$ if you choose. As with SALLI, you can use #include "sallio" after #include "sumit" in order to import the types commonly used for input and output.