|  |  4.14.3 number operations 
 
+addition
-negation or subtraction
*multiplication
/division
%,modmodulo
^,**power, exponentiation (by an integer)
<=, >=, ==, <>comparison
modinteger modulo (the remainder of the division div), always non-negative 
Note: Quotient and exponentiation is only recognized as a
number expression if it is already a number, see  Miscellaneous oddities.
For the behavior of comparison operators in rings with
ground field different from real or the rational numbers, see  boolean expressions.
 
Example:
 |  |   ring r=0,x,dp;
  number n = 1/2 +1/3;
  n;
==> 5/6
  n/2;
==> 5/12
  1/2/3;
==> 1/6
  1/2 * 1/3;
==> 1/6
  n = 2;
  n^-2;
==> 1/4
  // the following oddities appear here
  2/(2+3);
==> // ** int division with `/`: use `div` instead in line >>  2/(2+3);<<
==> 0
  number(2)/(2+3);
==> 2/5
  2^-2; // for int's exponent must be non-negative
==>    ? exponent must be non-negative
==>    ? error occurred in or before ./examples/number_operations.sing line 1\
   2: `  2^-2; // for int's exponent must be non-negative`
  number(2)^-2;
==> 1/4
  3/4>=2/5;
==> 1
  2/6==1/3;
==> 1
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