Davidshw

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thanks for your reply. 
the purpose for this question is to do transfer function expression approximation for symbolic circuit analysis. such as description in this link
https://reference.wolfram.com/applications/insydes/Tutorial/IntroductiontoSymbolicApproximation.html
because there may be many , such as hundreds of sub-expression in each plus items in a transfer function, like this
H=( ( (exprA1+exprA2+…) * (exprB1+exprB2+…) )/( (exprC1+exprC2+…)* (exprD1+exprD2+…) )
In a plus item, such as (exprA1+exprA2+…), each sub-expression is a product of circuit parameters, the number of sub-expressions may large (hundreds of), but in a circuit typical design point, the value of most of these sub-exrpessions is much smaller than the others, they can be droped.
For example, if at a circuit’s typical design point, eval each sub-expression, we have
Abs(exprA1) >> abs(exprA2), abs(exprA3), …, abs(exprAN), 
Then the plus item (exprA1+exprA2+…) can be approximately be (exprA1), so the transfer function is more simple for a circuit designer.
To do symbolic approximation, I need to find all the position in DAG of these sub-expression, estimate each subexpression at design point in the transfer function, if its value is smaller than a threshold, this sub-expression can be substituted with 0.

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