define: Define variables

  Syntax

    'tal:define' syntax::

      argument       ::= define_scope [';' define_scope]*
      define_scope   ::= (['local'] | 'global') define_var
      define_var     ::= variable_name expression
      variable_name  ::= Name

    *Note: If you want to include a semi-colon (;) in an 'expression',
    it must be escaped by doubling it (;;).*

  Description

    The 'tal:define' statement defines variables.  You can define two
    different kinds of TAL variables: local and global.  When you
    define a local variable in a statement element, you can only use
    that variable in that element and the elements it contains.  If
    you redefine a local variable in a contained element, the new
    definition hides the outer element's definition within the inner
    element.  When you define a global variables, you can use it in
    any element processed after the defining element.  If you redefine
    a global variable, you replace its definition for the rest of the
    template.

    *Note: local variables are the default*

    If the expression associated with a variable evaluates to
    *nothing*, then that variable has the value *nothing*, and may be
    used as such in further expressions. Likewise, if the expression
    evaluates to *default*, then the variable has the value *default*,
    and may be used as such in further expressions.

  Examples

    Defining a global variable::

      tal:define="global company_name string:Zope Corp, Inc."

    Defining two variables, where the second depends on the first::

      tal:define="mytitle template/title; tlen python:len(mytitle)"