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( returns negative value on error )
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: _ ( c -- n )
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( '0' is ASCII 48 )
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48 -
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DUP 0< IF EXIT THEN ( bad )
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DUP 10 < IF EXIT THEN ( good )
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( 'a' is ASCII 97. 59 = 97 - 48 )
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49 -
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DUP 0< IF EXIT THEN ( bad )
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DUP 6 < IF 10 + EXIT THEN ( good )
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( bad )
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255 -
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;
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