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(1) DefInes a data world of NTOT entries
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(2) Breaks up world into equal parts in each processor decomposed in one dimension
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(3) Instructs that PHINEW AND PHIOLD are aligned exactly with world i.e. PHINEW(I) and PHIOLD(I) are stored in the I'th position of world
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(4) EOSHIFT is "End-Off" SHIFT. The calls shift PHIOLD by one both to left and right. The indices take care of WORLD (1 and NTOT) being boundary values
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EOSHIFT (and corresponding circular shift (CSHIFT) are "standard" Fortran90 array manipulation routines. They are not specific to parallel computing. We can write this statement (4) as a direct array operation.
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BEGIN PHINEW(2:NTOT1) = 0.5 * (PHIOLD (1:NTOT2) + PHIOLD (3:NTOT))
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(5) Forms TEST as maximum change in absolute value of j at any world location. MAXVAL is a Fortran90 intrinsic function.
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NOTE: Any subscript n:m is an "array section" or set of indices
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