format "%-12(scrub(amount))"
<<<
>>>1
--- Context is first posting of the following transaction ---
2004/05/27 Book Store
    Expenses:Books                 20 BOOK @ $10
    Liabilities:MasterCard        $-200.00

--- Input format string ---
%-12(scrub(amount))

--- Format elements ---
Element:    EXPR  flags: 0x1  min: 12  max:  0  expr: scrub(amount)

--- Formatted string ---
"20 BOOK     "
>>>2
=== 0
format "%12(scrub(amount))"
>>>1
--- Context is first posting of the following transaction ---
2004/05/27 Book Store
    Expenses:Books                 20 BOOK @ $10
    Liabilities:MasterCard        $-200.00

--- Input format string ---
%12(scrub(amount))

--- Format elements ---
Element:    EXPR  flags: 0x0  min: 12  max:  0  expr: scrub(amount)

--- Formatted string ---
"     20 BOOK"
>>>2
=== 0