In broadest terms, by some means you give an ordering of infix operators, and then that ordering is used to determine the grouping of operators (as you well know?). I'm not sure why you need a special sei to give such information; perhaps the thought was that a computer would be able to spot the tag and then parse and apply such rules. The order of operators is not set in the grammar, so this is purely a matter of interpretation. Concretely I would give these examples.
li pa su'i re pi'i ci du li so
1+2 * 3 = (1+2) * 3 = 9
ti'o zo pi'i ce'o zo su'i bi'e zei porsi li pa su'i re pi'i ci du li ze
1+2 * 3 = 1+ (2 * 3) = 7
If you restricted the contents of the ti'o clause to be of the form
zo X ce'o zo Y ce'o zo Z ....... bi'e zei porsi
or
zo X ce'o zo Y ce'o zo Z ....... bi'e zei fatporsi
then one may be able to have a program parse this quite easily.
-- Ross