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dc, an arbitrary precision calculator

10. Miscellaneous

`!'

Will run the rest of the line as a system command. Note that parsing of the !<, !=, and !> commands take precidence, so if you want to run a command starting with <, =, or > you will need to add a space after the !.

`#'

Will interpret the rest of the line as a comment. (This command is a GNU extension.)

`:r'

Will pop the top two values off of the stack. The old second-to-top value will be stored in the array r, indexed by the old top-of-stack value.

`;r'

Pops the top-of-stack and uses it as an index into the array r. The selected value is then pushed onto the stack.

Note that each stacked instance of a register has its own array associated with it. Thus `1 0:a 0Sa 2 0:a La 0;ap' will print 1, because the 2 was stored in an instance of 0:a that was later popped.