Yorick: Define variable

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### 1.1.1 Defining a variable The following five Yorick statements define the five variables `c`, `m`, `E`, `theta`, and `file`:
    c = 3.00e10;  m = 9.11e-28
    E = m*c^2
    theta = span(0, 6*pi, 200)
    file = create("damped.txt")
Variable names are case sensitive, so `E` is not the same variable as `e`. A variable name must begin with a letter of the alphabet (either upper or lower case) or with an underscore (`_`); subsequent characters may additionally include digits. A semicolon terminates a Yorick statement, so the first line contains two statements. To make Yorick statements easier to type, you don't need to put a semicolon at the end of most lines. However, if you are composing a Yorick program in a text file (as opposed to typing directly to Yorick itself, a semicolon at the end of every line reduces the chances of a misinterpretation, and makes your program easier to read. Conversely, a new line need not represent the end of a statement. If the line is incomplete, the statement automatically continues on the following line. Hence, the second and third lines above could have been typed as:
    E=
      m *
      c^2
    theta= span(0, 6*pi,
                200)
In the second line, `*` and `^` represent multiplication and raising to a power. The other common arithmetic operators are `+`, `-`, `/` (division), and `%` (remainder or modulo). The rules for forming arithmetic expressions with these operators and parentheses are the same in Yorick as in Fortran or C (but note that `^` does not mean raise to a power in C, and Fortran uses the symbol `**` for that operation). The `span` function returns 200 equally spaced values beginning with `0` and ending with `6*pi`. The variable `pi` is predefined as 3.14159... The `create` function returns an object representing the new file. The variable `file` specifies where output functions should write their data. Besides numbers like `c`, `m`, and `E`, or arrays of numbers like `theta`, or files like `file`, Yorick variables may represent several other sorts of objects, taken up in later chapters. The `=` operator is itself a binary operator, which has the side effect of redefining its left operand. It associates from right to left, that is, the rightmost `=` operation is performed first (all other binary operators except `^` associate from left to right). Hence, several variables may be set to a single value with a single statement:
    psi = phi = theta = span(0, 6*pi, 200)
When you define a variable, Yorick forgets any previous value and data type:
    phi = create("junk.txt")