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 -=                  Subtraction compound assignment operator (binary)
------------------------------------------------------------------------------
 Syntax
   xVar -= xValue

 Arguments
   xVar is a numeric or string variable that receives the assignment. The
   variable must be initialized to a value before performing the operation.

   xValue is the right operand to the - operator.

 Description
   The -= operator performs a - operation with the current value
   of xVar and xValue, then assigns the result to xVar.

   The action by -= is equivalent to xVar := ( xVar - xValue ).

   The -= operator, when applied to two character strings, trims any
   trailing spaces from the left string and concatenates the right
   string.

 Example
   #define EXAMPLE_OPERATOR
   #include example.hdr

   proc Test_938
   vardef
      uint nValue
   enddef
   nValue := 220
   nValue -= 20
   ? nValue              // prints 200
   endproc

   proc main
   Test_938()
   endproc