double precision function kutl_mass2(w1, w2)
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*  Find the invariant mass of w1 and w2. No position information is used.
*  If mass^2 < 0, then a negative value is returned.
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*  Input Parameters:
*  w1(10)       double precision array
*               Input w track 1
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*  w2(10)       double precision array
*               Input w track 2
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*  Output Parameters:
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*  Return value: Invariant mass of w1, w2
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*  Other routines:
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*  Notes:
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*  Author:   Paul R Avery      Created:  Mon Jul 28 05:32:22 EDT 1997
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*  Major revisions:
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