m4: Len

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1 11.1 Calculating length of strings
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1 The length of a string can be calculated by 'len':
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1  -- Builtin: len (STRING)
1      Expands to the length of STRING, as a decimal number.
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1      The macro 'len' is recognized only with parameters.
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1      len()
1      =>0
1      len(`abcdef')
1      =>6
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