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string operators with require string arguments, mixed operators,
which can work with different types and special operators (only "defined"
for now).

intop: /, *, - , ||, &&, &, |, <, >, <=, >=, a.s.o.
strop: strlen, strempty, eq, ne, a.s.o.
mixed ops:  +, ==, !=
special op: defined



1. "Compile" Time (sip-router start time)
-----------------------------------------

The integer only and string only operators require that all of their operands
have the type of the operator:

intop operands  (o1 <intop> o2):
        int     str     unknown
int     OK      err     OK
str     err     err     err
unknown OK      err     OK

strop operands (o1 <strop> o2):
        int     str     unknown
int     err     err     err
str     err     OK      OK
unknown err     OK      OK

The mixed operators require that all of their operands have the _same_
type, with the exception of '+' which will allow an int operand on the right
side, even if the left side is a string.

mixed ops:

+:
        int     str     unknown
int     OK      err     OK
str     OK*     OK      OK
unknown OK      OK      OK

==/!=:

        int     str     unkown
int     OK      err     OK
str     err     OK      OK
unkown  OK      OK      OK



2. Runtime
-----------

The main difference is that we don't have unknown types anymore, since we
 can resolve the avps & pvars => everything has a type: int, str or undefined.

everything is allowed (no error), by automatically converting to the
type required by the operator.
All integer operators convert their operands to int and all string operators
to str.

Conversion table:
(int I, str S)
        int     str:"I" str:S   undefined
(int)   I       I       0       0

where str:"I" means the str is an interger between quotes, and str:S
 means the string is not number.

        int     str undefined
(str)   "I"     S   ""


Special cases: +, == and !=

The "mixed" operators that support both int & str in the same time try to
determine the type of the operation by looking at the left operand.
If the left operand is int, the right one will be converted to int too
(using the above conversion table) and then the operation will be performed
Same for string.

 o1 <op> o2 => o1 <type_of(o1) op> (type_of(o1)) (o2).

undef in the left side is handled in a special way: for == and !=, the type
is taken from the right operand:
 undef <cmpop> o2 => (type_of(o2)) undef <type_of(o2) cmpop> o2

For +, undef in the left side is considered to be string:
 undef <+> o2 => (str) undef <s.> (str)o2 => "" <s.> (str)o2 => (str)o2


==/!=   i2                      s2                  undef
i1      i1 <intop> i2           i1 <intop>(int)s2   i1 <intop> 0
s1      s1 <strop> (str)i2      s1 <strop> s2       s1 <strop> ""
undef   0 <intop> i2            "" <strop> s2       "" <strop> ""

(where <intop> is the integer version of the operator and <strop> is the
 string version)

+       i2                      s2                  undef
i1      i1 <i+> i2              i1 <i+>(int)s2      i1 <i+> 0
s1      s1 <s.> (str)i2         s1 <s.> s2          s1 <s.> ""
undef   "" <s.> (str)i2         "" <s.> s2          "" <s.> ""

(<i+> is interger  +, <s.> is string concat)

Note: the undef in the left side case is handled differently for +.


== table of truth:

==      int         str:""      str:"I2"    str:S       undef
int     I1==I2      I1==0       I1==I2      I1==0       I1==0
str     S1=="I1"    S1==""      S1=="I2"    S1==S       S1==""
undef   0==I2       true        ""=="I2"    ""==S       true


3. Converting to int or string
-------------------------------

For now we don't have any explicit conversion operators (might be introduced),
so we have to reuse '+'.

Converting $v to int:   (0 + $v)
Converting $v to str:   ("" + $v)

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