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"Abstract supertype of types that support scaling by a 
 numeric factor. Examples of such types include vectors and 
 matrices. The _scale_ operator `a ** x` accepts a scale 
 factor as its first operand, and an instance of `Scalable` 
 as its second operand.
 
     Vector scaled = 2.0 ** Vector(x,y,z);
 
 Concrete classes which implement this interface should
 satisfy:
 
 - `x == 1**x`
 - `a ** (b ** x) == a*b ** x`
 
 where `1` denotes the multiplicative identity of the 
 numeric scaling type.
 
 Concrete classes which implement this interface are
 encouraged to also satisfy [[Invertible]], in which case, 
 the following identity should be satisfied:
 
 - `-x == -1**x`
 
 Concrete classes which implement this interface and which
 also satisfy [[Summable]] should satisfy:
 
 - `x+x == 2**x`
 - `a ** (x+y) == a**x + a**y`
 
 The [[scaling type|Scale]] must be a [[numeric|Numeric]]
 type, but is not required to be [[Number]], since a complex
 number scaling type should be allowed."
by ("Gavin")
tagged("Numbers")
shared interface Scalable of Value 
        given Value satisfies Scalable 
        given Scale satisfies Numeric {
    
    "Scale this value by the given scale factor."
    shared formal Value scale(Scale scalar);
    
}