The following logical operators are allowed on vectors and scalers
in SM, where non-zero means true:

Unary:
   !expr          Logical complement

Binary:
   expr == expr   Equal to                 expr != expr   Not equal
   expr < expr    Less than                expr <= expr   Less than or equal
   expr > expr    Greater than             expr >= expr   Greater than or equal
   expr && expr   Logical and              expr || expr   Logical or

Only == and != are allowed for string valued vectors. All arithmetic vectors
test unequal to all string-valued vectors.

Use `HELP arithmetic' for the arithmetic operators.  You can test to
see if a variable is defined by looking at the value of $?var (see
DEFINE).