When given --infer-equity, hledger will change transaction prices to balancing equity postings. This introduces the inverse operation, --infer-costs, which will match balancing equity postings and transform them into a transaction price, allowing --cost to work properly with them. This is only a partial inverse as it needs to use some heuristics to match the postings which will not work in complicated cases.
Specifically, when hledger finds exactly two conversion postings in a transaction (by default, subaccounts of equity:conversion or equity:trad(e|ing)), it will find the first posting in the transaction whose amount is negative one of the conversion posting amounts, and inserts the corresponding transaction price.
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