This prefix operator is used to represent n × m matrices. MAT has n arguments interpreted as rows of the matrix, each of which is a list of m expressions representing elements in that row. For example, the matrix
would be written as mat((a,b,c),(d,e,f)).
Note that the single column matrix
becomes mat((x),(y)). The inside parentheses are required to distinguish it from the single row matrix
that would be written as mat((x,y)).