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# isSymmetricFDivisor -- checks whether a symmetric divisor intersects all the F-curves nonnegatively

- Usage:
    `isSymmetricFDivisor(D)`
- Inputs:
  - D, an instance of the type SymmetricDivisorM0nbar,
- Outputs:
  - b, a Boolean value,

## Description

We say a symmetric divisor on $\bar{M}_{0,n}$ is a symmetric F-divisor if $D . F_{I_1,I_2,I_3,I_4} \geq 0$ for every F curve.

In the example below, we see that for $n=8$, the divisor $3B_2+2B_3+4B_4$ is a symmetric F-divisor, while the divisor $B_2$ is not.

    i1 : D=symmetricDivisorM0nbar(8,3*B_2+2*B_3+4*B_4)

    o1 = 3*B  + 2*B  + 4*B
            2      3      4

    o1 : S_8-symmetric divisor on M-0-8-bar

    i2 : isSymmetricFDivisor(D)

    o2 = true

    i3 : D=symmetricDivisorM0nbar(8,B_2)

    o3 = B
          2

    o3 : S_8-symmetric divisor on M-0-8-bar

    i4 : isSymmetricFDivisor(D)
    This divisor has negative intersection with the F curve F_{3, 2, 2, 1} (and maybe others too)

    o4 = false

## Ways to use isSymmetricFDivisor:

- `isSymmetricFDivisor(SymmetricDivisorM0nbar)`

## For the programmer

The object isSymmetricFDivisor is a method function.

The source of this document is in /build/macaulay2-88fgJW/macaulay2-1.25.11+ds/M2/Macaulay2/packages/ConformalBlocks.m2:1267:0.