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MapDown

convert a constant polynomial to a number
Syntax 

MapDown(F:POLY):RAT or ZMOD

Description
This function converts a constant polynomial to the equivalent coefficient. If the argument is not a constant polynomial, an error is signalled.

Example 
  Use QQ[x,y,z];
  Type((x+1)^2 - x*(x+2));    -- value is seen as a polynomial
POLY
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  MapDown((x+1)^2 - x*(x+2)); -- attempt to map down to the coeff ring
1
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  Type(It);                   -- value is now simply a coefficient
RAT
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  MapDown((x+1)^2 - x^2);     -- 2*x + 1 is not a coefficient
ERROR: Cannot MapDown non-const poly
CONTEXT: Error("Cannot MapDown non-const poly")
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