Pressing a digit or other numeric key begins numeric entry using the
minibuffer. The number is pushed on the stack when you press the `RET`
or `SPC` keys. If you press any other non-numeric key, the number is
pushed onto the stack and the appropriate operation is performed. If
you press a numeric key which is not valid, the key is ignored.

There are three different concepts corresponding to the word "minus,"
typified by a-b (subtraction), -x
(change-sign), and -5 (negative number). Calc uses three
different keys for these operations, respectively:
`-`, `n`, and `_` (the underscore). The `-` key subtracts
the two numbers on the top of the stack. The `n` key changes the sign
of the number on the top of the stack or the number currently being entered.
The `_` key begins entry of a negative number or changes the sign of
the number currently being entered. The following sequences all enter the
number *-5* onto the stack: `0 RET 5 -`,

Some other keys are active during numeric entry, such as `#` for
non-decimal numbers, `:` for fractions, and `@` for HMS forms.
These notations are described later in this manual with the corresponding
data types. See section Data Types.

During numeric entry, the only editing key available is `DEL`.

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