Created Environment (markdown)

Environment.md

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+The `$MPW/Environment.text` file contains environment variables used by some MPW programs. Particularly, header file locations are defined here.
+
+The file format is:
+
+    # comment
+    name = value
+    name += value
+    name ?= value
+
+Name may contains alphanumerics and underscores.  
+
+* `=` assigns the value, overwriting any previous value.
+* `+=` appends the value to any previous value.
+* `?=` assigns the value if and only if the variable was not already assigned. 
+
+## Variable Expansion
+
+Variables are expanded within the value.  Recognized forms are:
+
+* `$name`
+* `${name}`
+* `{name}`
+
+
+## Command line 
+
+Variables can also be defined as part of the mpw command. eg:
+    mpw -DMPWVersion=3.5 SC ...
+
+These variables are processed after internal variables are set (`$Command`) but before the `Environment.text` file is loaded.