Value expression.

[prataprc]

There are many places in ledger-cli where value expression is used. This issues tracks all such places in case we decide to implement:

• --period-sort VEXPR Sort postings within each period according to VEXPR.
• --limit EXPR Limit which postings are used in calculations by EXPR.
• --amount EXPR Change value expression reported in register report.
• --total VEXPR Change the value expression used for “totals” column in register and balance reports.
• --sort VEXPR Sort a report using VEXPR.