Calculates conditional requirements for multiple requirements
        at once. This is a shorthand to be reduce duplication and to
        keep an inline declarative syntax. For example:

            lib x : x.cpp : [ conditional <toolset>gcc <variant>debug :
                <define>DEBUG_EXCEPTION <define>DEBUG_TRACE ] ;

def conditional(self, condition, requirements):
        """Calculates conditional requirements for multiple requirements
        at once. This is a shorthand to be reduce duplication and to
        keep an inline declarative syntax. For example:

            lib x : x.cpp : [ conditional <toolset>gcc <variant>debug :
                <define>DEBUG_EXCEPTION <define>DEBUG_TRACE ] ;
        """
        assert is_iterable_typed(condition, basestring)
        assert is_iterable_typed(requirements, basestring)
        c = string.join(condition, ",")
        if c.find(":") != -1:
            return [c + r for r in requirements]
        else:
            return [c + ":" + r for r in requirements]