The mathematic notation bit may be sound, except for the bit about "...being followed by a letter."

And being a member of the Cabal of Fortran Programmers, no Fortran programmer would ever consider '-' to be a reasonable character to permit in a variable name. Code like

IF( IF .NE. NE .OR. NE .EQ. OR) THEN(IF, OR, NE)
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is bad enough; allowing code like

IF(IF .NE. IF - NE) .OR. IF-NE .NE. NE) THEN(IF, NE, IF-NE)
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where IF - NE is an arithmetic operation and IF-NE is a variable would be insufferable.