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# isEffectiveExpression(DivisorClassRepresentativeM0nbar) -- determines whether a divisor class expression is effective

- Function: isEffectiveExpression
- Usage:
    `isEffectiveExpression(D)`
- Inputs:
  - D, an instance of the type DivisorClassRepresentativeM0nbar,
- Outputs:
  - a Boolean value,

## Description

This function tests whether the coefficients in an object of type DivisorClassRepresentativeM0nbar are all nonnegative.

Warning: this function tests only the input representative of a divisor class. That is, it answers whether the input is an effective expression, not whether there exists an effective expression equivalent to the input.

    i1 : L1= { {{3,1},-1}, {{1,4},1} };

    i2 : D1=divisorClassRepresentativeM0nbar(6,L1)

    o2 = DivisorClassRepresentativeM0nbar{"DivisorExpression" => HashTable{{1, 3} => -1}}
                                                                           {1, 4} => 1
                                          "NumberOfMarkedPoints" => 6

    o2 : DivisorClassRepresentativeM0nbar

    i3 : isEffectiveExpression(D1)

    o3 = false

    i4 : L2= { {{3,1},1}, {{1,4},1} };

    i5 : D2=divisorClassRepresentativeM0nbar(6,L2)

    o5 = DivisorClassRepresentativeM0nbar{"DivisorExpression" => HashTable{{1, 3} => 1}}
                                                                           {1, 4} => 1
                                          "NumberOfMarkedPoints" => 6

    o5 : DivisorClassRepresentativeM0nbar

    i6 : isEffectiveExpression(D2)

    o6 = true

## Ways to use this method:

- isEffectiveExpression(DivisorClassRepresentativeM0nbar) -- determines whether a divisor class expression is effective

The source of this document is in /build/macaulay2-88fgJW/macaulay2-1.25.11+ds/M2/Macaulay2/packages/M0nbar.m2:1938:0.