spec: Add variable declaration

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+---
+title: Variable declaration
+version: 1.0.0
+state: draft
+---
+
+# Variable declaration
+
+## Terms
+
+list: multiple tokens seperated by spaces  
+token: a particular fully formed symbol or set of symbols representing a specific meaning in rabbitscript and that has no more specific underlying meaning. (i.e. a VARNAME is a token but not an EXPRESSION, however a LITERAL or an OPERAND are tokens)  
+meaning: something that a parser of the grammar could act on to verify syntaxic validity (is a token valid in relation to the token that follows or precedes it)  
+representation: something that defines meaning and that a parser of the grammar could act on to verify a token's validity (is a token properly represented or does it contain a symbol that breaks the validity of its representation)  
+
+## Grammar
+
+### Declaration
+
+```
+VARNAME = EXPRESSION
+```
+
+### VARNAME
+
+A VARNAME is represented by a series of alphanumeric characters:
+
+```
+alphanum+
+```
+
+### EXPRESSION
+
+```
+(OPERATION | LITERAL)
+```
+
+### LITERAL
+
+A literal is represented by a set of alphanumeric symbols. If the symbols can be parsed as a number then the type is a number, otherwise it is a a string.
+
+```
+(number|string)
+```
+
+### OPERATION
+
+Operations are calculations on numbers using reverse polish notation.
+
+An operation is defined as a list of n operands (where n>1) followed by a a list of n-1 operators, or otherwise said an operand followed by n>=1 operands followed by n operators:
+
+```
+OPERAND (OPERAND ){N} OPERATOR{N}
+```
+
+An operand is defined strictly as a number (integer or floating). An operator is defined by the addition, subtraction, multiplication and division symbols as follows:
+
+```
+(+|-|\*|/)
+```
+
+
+## Example
+
+Declares a variable `a` with the number value 15.
+
+```
+a=15
+```
+
+Declares a variable `b` with the string value "a string".
+
+```
+s=a string
+```
+
+Declares the variable `o` that is an operation resulting in the number value 6.
+
+```
+o=1 5 +
+```
+
+The same as above can be specified with an arbitrary number of spaces around the tokens. They should be trimmed by parsers:
+
+```
+o = 1 5 +
+```