  This paper describes a new set of block source codes well suited for data
compression. These codes are defined by sets of productions rules of the form
a.l->b, where a in A represents a value from the source alphabet A and l, b are
-small- sequences of bits. These codes naturally encompass other Variable
Length Codes (VLCs) such as Huffman codes. It is shown that these codes may
have a similar or even a shorter mean description length than Huffman codes for
the same encoding and decoding complexity. A first code design method allowing
to preserve the lexicographic order in the bit domain is described. The
corresponding codes have the same mean description length (mdl) as Huffman
codes from which they are constructed. Therefore, they outperform from a
compression point of view the Hu-Tucker codes designed to offer the
lexicographic property in the bit domain. A second construction method allows
to obtain codes such that the marginal bit probability converges to 0.5 as the
sequence length increases and this is achieved even if the probability
distribution function is not known by the encoder.
