1. For encoding a symbol sequence, a method comprising a sequence of at least one operation of which at least one is an entropy-pump operation, which includes: A) receiving as the operation's input a sequence of symbols drawn from a symbol alphabet; B) dividing the operation's input into a sequence of blocks whose values belong to a block-value dictionary such that the block-value dictionary's block values are so classified into enumerative classes that: i) at least one enumerative class includes more than one of the block values; and ii) a respective index value is so associated with each block value that belongs to an enumerative class to which more than one block value belongs that some block values that belong to different enumerative classes have the same index value but each block value's index value identifies it uniquely within the enumerative class to which it belongs: C) generating a classifying output component by an operation that, at least if a predetermined coding criterion is met, includes entropy encoding a coding input derived from a sequence of class identifiers that specify the enumerative-class sequence of enumerative classes to which successive blocks in the input sequence belong; and D) for each enumerative class that belongs to the class sequence to which more than one of the block value's dictionary values belongs, generating as an index output component a sequence of indexes of the values of the input sequence's successive blocks whose values belong to that enumerative class, the indexes being so associated with block values that some block values that belong to different enumerative classes have the same index value but each block value's index value identifies it uniquely within the enumerative class to which it belongs.