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sub 

        # BigDecimal.sub

(from ruby core)
---
    sub(value, digits)  -> bigdecimal

---

Subtract the specified value.

e.g.
    c = a.sub(b,n)

digits
:   If specified and less than the number of significant digits of the
    result, the result is rounded to that number of digits, according to
    BigDecimal.mode.

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