A new class for sparse matrices stored in symmetric sparse row format
Objects from the Class
Objects can be created by calls of the form new("matrix.ssr", ...).
Slots
ra:
Object of class numeric, a real array of nnz elements containing the non-zero
elements of the lower triangular part of A, stored in row order. Thus, if i<j, all elements
of row i precede elements from row j. The order of elements
within the rows is immaterial.
ja:
Object of class integer, an integer array of nnz elements containing the column
indices of the elements stored in ‘ra’.
ia:
Object of class integer, an integer array of n+1 elements containing pointers to
the beginning of each row in the arrays ‘ra’ and ‘ja’. Thus
‘ia[i]’ indicates the position in the arrays ‘ra’ and ‘ja’
where the ith row begins. The last, (n+1)st, element of ‘ia’
indicates where the n+1 row would start, if it existed.
providing 
.