convert(expr, confrac)
convert(expr, confrac, maxit)
convert(expr, confrac, 'cvgts' )
convert(expr, confrac, maxit, 'cvgts')
convert(expr, confrac, 'subdiagonal')
convert(expr, confrac, var)
convert(expr, confrac, var, ctype)
convert(expr, confrac, var, order)
convert(expr, confrac, var, order, 'subdiagonal')